diff --git a/README.md b/README.md
index 91f0d58..bf8344e 100644
--- a/README.md
+++ b/README.md
@@ -32,6 +32,7 @@ Financial은 개인 금융 데이터를 서버 렌더링 PHP 화면에서 관리
 - `app/lib/auth.php`: 인증, remember token, CSRF, no-store header
 - `app/lib/db.php`: PDO 연결
 - `app/lib/account_service.php`: 계좌 서비스
+- `app/lib/transaction_record_service.php`: 거래 fingerprint, 입력 검증, 공통 거래 row 기록
 - `app/lib/transaction_service.php`: 거래 저장과 조회
 - `app/lib/installment_service.php`: 할부 청구와 선납 처리
 - `app/lib/loan_service.php`: 대출과 상환 처리
@@ -58,9 +59,12 @@ Financial은 개인 금융 데이터를 서버 렌더링 PHP 화면에서 관리
 4. 저장 후 redirect와 flash 메시지로 응답합니다.
 5. 카테고리 추천 API는 merchant rule 우선순위와 confidence를 반환합니다.
 
+거래 테이블에 실제 row를 추가하는 공통 경로는 `transaction_record_service.php`입니다. 일반 거래 등록/수정은 같은 입력 검증 함수를 쓰고, 대출 실행/상환/중도상환과 할부 선결제도 공통 기록 함수를 통해 `transactions`에 남깁니다. 카드/계좌/할부/대출 기능을 바꿀 때는 먼저 이 공통 기록 규칙과 잔액 재계산 흐름을 확인해야 합니다.
+
 ## 주요 함수/모듈
 
 - `auth.php`: 로그인, 세션, CSRF
+- `transaction_record_service.php`: 거래 공통 검증과 row 기록
 - `transaction_service.php`: 거래 저장과 조회
 - `installment_*`: 할부 청구와 선납 처리
 - `merchant_rules.php`: 가맹점 자동분류 규칙 관리
diff --git a/app/lib/installment_service.php b/app/lib/installment_service.php
index 0d12004..440415e 100644
--- a/app/lib/installment_service.php
+++ b/app/lib/installment_service.php
@@ -1,8 +1,9 @@
 <?php
 
-require_once __DIR__ . '/db.php';
-require_once __DIR__ . '/account_service.php';
-require_once __DIR__ . '/card_billing_service.php';
+require_once __DIR__ . '/db.php';
+require_once __DIR__ . '/account_service.php';
+require_once __DIR__ . '/card_billing_service.php';
+require_once __DIR__ . '/transaction_record_service.php';
 
 function split_amount_evenly(float $amount, int $months): array
 {
@@ -547,43 +548,10 @@ function prepay_installment(
 
         recalculate_installment_status($installmentId);
 
-        $stmt = $pdo->prepare("
-            INSERT INTO transactions
-            (
-                user_id,
-                account_id,
-                category_id,
-                transaction_type,
-                amount,
-                is_installment,
-                installment_months,
-                installment_interest_rate,
-                installment_interest_total,
-                installment_total_billed,
-                installment_prepay_amount,
-                transaction_date,
-                merchant_name,
-                description,
-                related_account_id,
-                fingerprint
-            )
-            VALUES (?, ?, ?, 'expense', ?, 0, NULL, 0, 0, NULL, ?, ?, ?, ?, NULL, ?)
-        ");
-
-        $desc = $description ?: '할부 선결제/중도상환';
-
-        $fingerprint = hash('sha256', implode('|', [
-            $userId,
-            $paymentAccountId,
-            'installment_prepay',
-            $installmentId,
-            $prepayDate,
-            number_format($totalAmount, 2, '.', ''),
-            $desc
-        ]));
-
-        $stmtCat = $pdo->prepare("
-            SELECT id
+        $desc = $description ?: '할부 선결제/중도상환';
+
+        $stmtCat = $pdo->prepare("
+            SELECT id
             FROM categories
             WHERE user_id = ?
               AND category_type = 'expense'
@@ -594,22 +562,23 @@ function prepay_installment(
         $category = $stmtCat->fetch();
 
         if (!$category) {
-            throw new RuntimeException('선결제 기록용 expense 카테고리(기타지출)를 찾을 수 없습니다.');
-        }
-
-        $stmt->execute([
-            $userId,
-            $paymentAccountId,
-            (int)$category['id'],
-            $totalAmount,
-            $totalAmount,
-            $prepayDate,
-            $installment['merchant_name'],
-            '[할부 선결제] ' . $desc,
-            $fingerprint
-        ]);
-
-        $pdo->commit();
+            throw new RuntimeException('선결제 기록용 expense 카테고리(기타지출)를 찾을 수 없습니다.');
+        }
+
+        insert_transaction_record([
+            'user_id' => $userId,
+            'account_id' => $paymentAccountId,
+            'category_id' => (int)$category['id'],
+            'transaction_type' => 'expense',
+            'amount' => $totalAmount,
+            'transaction_date' => $prepayDate,
+            'merchant_name' => $installment['merchant_name'],
+            'description' => '[할부 선결제] ' . $installmentId . ' / ' . $desc,
+            'related_account_id' => null,
+            'installment_prepay_amount' => $totalAmount,
+        ], true);
+
+        $pdo->commit();
 
         recalculate_account_balance($paymentAccountId);
     } catch (Throwable $e) {
@@ -701,4 +670,4 @@ function rebuild_all_installments_for_user(int $userId): int
 
         throw $e;
     }
-}
\ No newline at end of file
+}
diff --git a/app/lib/loan_service.php b/app/lib/loan_service.php
index 591b5c5..5794dd9 100644
--- a/app/lib/loan_service.php
+++ b/app/lib/loan_service.php
@@ -1,7 +1,8 @@
 <?php
 
-require_once __DIR__ . '/db.php';
-require_once __DIR__ . '/account_service.php';
+require_once __DIR__ . '/db.php';
+require_once __DIR__ . '/account_service.php';
+require_once __DIR__ . '/transaction_record_service.php';
 
 function add_months_keep_day(string $date, int $months, int $paymentDay): string
 {
@@ -176,110 +177,10 @@ function get_category_id_by_name(int $userId, string $categoryType, string $name
     return $row ? (int)$row['id'] : null;
 }
 
-function build_tx_fingerprint(
-    int $userId,
-    int $accountId,
-    ?int $relatedAccountId,
-    int $categoryId,
-    string $transactionType,
-    float $amount,
-    string $transactionDate,
-    ?string $merchantName,
-    ?string $description,
-    ?int $sourceLoanId = null
-): string {
-    $raw = implode('|', [
-        $userId,
-        $accountId,
-        $relatedAccountId ?? 0,
-        $categoryId,
-        $transactionType,
-        number_format($amount, 2, '.', ''),
-        $transactionDate,
-        trim((string)$merchantName),
-        trim((string)$description),
-        $sourceLoanId ?? 0,
-    ]);
-
-    return hash('sha256', $raw);
-}
-
-function insert_transaction_row(array $data): int
-{
-    $pdo = db();
-
-    $sourceLoanId = !empty($data['source_loan_id']) ? (int)$data['source_loan_id'] : null;
-
-    $fingerprint = build_tx_fingerprint(
-        (int)$data['user_id'],
-        (int)$data['account_id'],
-        !empty($data['related_account_id']) ? (int)$data['related_account_id'] : null,
-        (int)$data['category_id'],
-        (string)$data['transaction_type'],
-        (float)$data['amount'],
-        (string)$data['transaction_date'],
-        $data['merchant_name'] ?? null,
-        $data['description'] ?? null,
-        $sourceLoanId
-    );
-
-    $stmt = $pdo->prepare("
-        SELECT id
-        FROM transactions
-        WHERE user_id = ?
-          AND fingerprint = ?
-        LIMIT 1
-    ");
-    $stmt->execute([
-        $data['user_id'],
-        $fingerprint
-    ]);
-    $exists = $stmt->fetch();
-
-    if ($exists) {
-        return (int)$exists['id'];
-    }
-
-    $stmt = $pdo->prepare("
-        INSERT INTO transactions
-        (
-            user_id,
-            account_id,
-            category_id,
-            transaction_type,
-            amount,
-            is_installment,
-            installment_months,
-            installment_interest_rate,
-            installment_interest_total,
-            installment_total_billed,
-            installment_prepay_amount,
-            transaction_date,
-            merchant_name,
-            description,
-            related_account_id,
-            source_loan_id,
-            fingerprint
-        )
-        VALUES (?, ?, ?, ?, ?, 0, NULL, 0, 0, NULL, 0, ?, ?, ?, ?, ?, ?)
-    ");
-
-    $stmt->execute([
-        $data['user_id'],
-        $data['account_id'],
-        $data['category_id'],
-        $data['transaction_type'],
-        $data['amount'],
-        $data['transaction_date'],
-        $data['merchant_name'],
-        $data['description'],
-        $data['related_account_id'],
-        $sourceLoanId,
-        $fingerprint,
-    ]);
-
-    return (int)$pdo->lastInsertId();
-}
+function insert_transaction_row(array $data): int
+{
+    return insert_transaction_record($data, true);
+}
 
 function create_loan_schedule_only(
     int $loanId,
@@ -928,20 +829,44 @@ function prepay_loan(
             )
             VALUES (?, ?, NULL, ?, ?, ?, ?, ?, ?, 'prepayment', 0, ?)
         ");
-        $stmt->execute([
-            $userId,
-            $loanId,
-            $accountId ?: null,
+        $stmt->execute([
+            $userId,
+            $loanId,
+            $accountId ?: null,
             $paymentDate,
             $principalAmount,
             $interestAmount,
             $feeAmount,
             $totalAmount,
-            $description,
-        ]);
-
-        if ($principalAmount > 0) {
-            $stmt = $pdo->prepare("
+            $description,
+        ]);
+
+        if (!empty($accountId) && $totalAmount > 0) {
+            $expenseCategoryId = get_category_id_by_name($userId, 'expense', '대출상환');
+
+            if ($expenseCategoryId) {
+                insert_transaction_row([
+                    'user_id' => $userId,
+                    'account_id' => $accountId,
+                    'category_id' => $expenseCategoryId,
+                    'transaction_type' => 'expense',
+                    'amount' => $totalAmount,
+                    'transaction_date' => $paymentDate,
+                    'merchant_name' => $loan['loan_name'],
+                    'description' => '[대출중도상환] 원금 ' .
+                        number_format($principalAmount, 0) .
+                        ' / 이자 ' .
+                        number_format($interestAmount, 0) .
+                        ' / 수수료 ' .
+                        number_format($feeAmount, 0),
+                    'related_account_id' => null,
+                    'source_loan_id' => $loanId,
+                ]);
+            }
+        }
+
+        if ($principalAmount > 0) {
+            $stmt = $pdo->prepare("
                 UPDATE loans
                 SET current_principal_balance = GREATEST(current_principal_balance - ?, 0)
                 WHERE id = ?
@@ -1151,4 +1076,4 @@ function update_loan_and_rebuild_full_history(int $userId, int $loanId, array $d
         }
         throw $e;
     }
-}
\ No newline at end of file
+}
diff --git a/app/lib/transaction_record_service.php b/app/lib/transaction_record_service.php
new file mode 100644
index 0000000..539ae7e
--- /dev/null
+++ b/app/lib/transaction_record_service.php
@@ -0,0 +1,274 @@
+<?php
+
+require_once __DIR__ . '/db.php';
+require_once __DIR__ . '/account_service.php';
+require_once __DIR__ . '/card_billing_service.php';
+
+function build_transaction_fingerprint(
+    int $userId,
+    int $accountId,
+    ?int $relatedAccountId,
+    int $categoryId,
+    string $transactionType,
+    float $amount,
+    string $transactionDate,
+    ?string $merchantName,
+    ?string $description,
+    ?int $sourceLoanId = null
+): string {
+    $parts = [
+        $userId,
+        $accountId,
+        $relatedAccountId ?? 0,
+        $categoryId,
+        $transactionType,
+        number_format($amount, 2, '.', ''),
+        $transactionDate,
+        trim((string)$merchantName),
+        trim((string)$description),
+    ];
+
+    if ($sourceLoanId !== null) {
+        $parts[] = $sourceLoanId;
+    }
+
+    return hash('sha256', implode('|', $parts));
+}
+
+function get_account_for_transaction(int $userId, int $accountId): ?array
+{
+    $pdo = db();
+
+    $stmt = $pdo->prepare("
+        SELECT *
+        FROM accounts
+        WHERE id = ?
+          AND user_id = ?
+        LIMIT 1
+    ");
+    $stmt->execute([$accountId, $userId]);
+
+    $row = $stmt->fetch();
+    return $row ?: null;
+}
+
+function get_transaction_billing_year_month(int $userId, int $accountId, string $transactionDate): ?string
+{
+    $account = get_account_for_transaction($userId, $accountId);
+    return $account ? get_card_billing_year_month($account, $transactionDate) : null;
+}
+
+function transaction_category_type(string $transactionType): string
+{
+    if ($transactionType === 'income') {
+        return 'income';
+    }
+
+    if ($transactionType === 'expense') {
+        return 'expense';
+    }
+
+    return 'transfer';
+}
+
+function parse_money_input($value): float
+{
+    return (float)str_replace(',', '', trim((string)$value));
+}
+
+function build_transaction_payload_from_input(array $input, int $userId): array
+{
+    $pdo = db();
+
+    $transactionType = (string)($input['transaction_type'] ?? 'expense');
+    $accountId = (int)($input['account_id'] ?? 0);
+    $categoryId = (int)($input['category_id'] ?? 0);
+    $amount = parse_money_input($input['amount'] ?? 0);
+    $transactionDate = (string)($input['transaction_date'] ?? date('Y-m-d'));
+    $merchantName = trim((string)($input['merchant_name'] ?? ''));
+    $description = trim((string)($input['description'] ?? ''));
+    $relatedAccountId = !empty($input['related_account_id']) ? (int)$input['related_account_id'] : null;
+    $isInstallment = (int)($input['is_installment'] ?? 0);
+    $installmentMonths = trim((string)($input['installment_months'] ?? '')) !== ''
+        ? (int)$input['installment_months']
+        : null;
+    $installmentInterestRate = trim((string)($input['installment_interest_rate'] ?? '0')) !== ''
+        ? (float)$input['installment_interest_rate']
+        : 0.0;
+    $installmentInterestTotal = trim((string)($input['installment_interest_total'] ?? '')) !== ''
+        ? parse_money_input($input['installment_interest_total'])
+        : null;
+    $installmentTotalBilled = trim((string)($input['installment_total_billed'] ?? '')) !== ''
+        ? parse_money_input($input['installment_total_billed'])
+        : null;
+
+    if ($amount <= 0) {
+        throw new RuntimeException('금액은 0보다 커야 합니다.');
+    }
+
+    if (!in_array($transactionType, ['income', 'expense', 'transfer', 'card_payment'], true)) {
+        throw new RuntimeException('거래 유형이 올바르지 않습니다.');
+    }
+
+    if ($accountId <= 0) {
+        throw new RuntimeException('주 계좌/카드를 선택하세요.');
+    }
+
+    $stmt = $pdo->prepare("SELECT id FROM accounts WHERE id = ? AND user_id = ? AND is_active = 1 LIMIT 1");
+    $stmt->execute([$accountId, $userId]);
+    if (!$stmt->fetch()) {
+        throw new RuntimeException('선택한 주 계좌/카드를 사용할 수 없습니다.');
+    }
+
+    if ($categoryId <= 0) {
+        throw new RuntimeException('카테고리를 선택하세요.');
+    }
+
+    $stmt = $pdo->prepare("
+        SELECT category_type
+        FROM categories
+        WHERE id = ?
+          AND user_id = ?
+          AND is_active = 1
+        LIMIT 1
+    ");
+    $stmt->execute([$categoryId, $userId]);
+    $category = $stmt->fetch();
+    if (!$category) {
+        throw new RuntimeException('선택한 카테고리를 사용할 수 없습니다.');
+    }
+
+    if ((string)$category['category_type'] !== transaction_category_type($transactionType)) {
+        throw new RuntimeException('거래 유형과 카테고리 유형이 일치하지 않습니다.');
+    }
+
+    if (in_array($transactionType, ['transfer', 'card_payment'], true) && !$relatedAccountId) {
+        throw new RuntimeException('관련 계좌를 선택해야 합니다.');
+    }
+
+    if ($relatedAccountId) {
+        if ($relatedAccountId === $accountId) {
+            throw new RuntimeException('주 계좌와 관련 계좌는 같을 수 없습니다.');
+        }
+
+        $stmt = $pdo->prepare("SELECT id FROM accounts WHERE id = ? AND user_id = ? AND is_active = 1 LIMIT 1");
+        $stmt->execute([$relatedAccountId, $userId]);
+        if (!$stmt->fetch()) {
+            throw new RuntimeException('선택한 관련 계좌를 사용할 수 없습니다.');
+        }
+    }
+
+    if ($transactionType !== 'expense') {
+        $isInstallment = 0;
+        $installmentMonths = null;
+        $installmentInterestRate = 0.0;
+        $installmentInterestTotal = null;
+        $installmentTotalBilled = null;
+    }
+
+    return [
+        'user_id' => $userId,
+        'account_id' => $accountId,
+        'category_id' => $categoryId,
+        'transaction_type' => $transactionType,
+        'amount' => $amount,
+        'transaction_date' => $transactionDate,
+        'merchant_name' => $merchantName !== '' ? $merchantName : null,
+        'description' => $description !== '' ? $description : null,
+        'related_account_id' => $relatedAccountId,
+        'is_installment' => $isInstallment,
+        'installment_months' => $installmentMonths,
+        'installment_interest_rate' => $installmentInterestRate,
+        'installment_interest_total' => $installmentInterestTotal,
+        'installment_total_billed' => $installmentTotalBilled,
+    ];
+}
+
+function insert_transaction_record(array $data, bool $skipIfDuplicate = true): int
+{
+    $pdo = db();
+
+    $sourceLoanId = !empty($data['source_loan_id']) ? (int)$data['source_loan_id'] : null;
+    $fingerprint = build_transaction_fingerprint(
+        (int)$data['user_id'],
+        (int)$data['account_id'],
+        !empty($data['related_account_id']) ? (int)$data['related_account_id'] : null,
+        (int)$data['category_id'],
+        (string)$data['transaction_type'],
+        (float)$data['amount'],
+        (string)$data['transaction_date'],
+        $data['merchant_name'] ?? null,
+        $data['description'] ?? null,
+        $sourceLoanId
+    );
+
+    if ($skipIfDuplicate) {
+        $stmt = $pdo->prepare("
+            SELECT id
+            FROM transactions
+            WHERE user_id = ?
+              AND fingerprint = ?
+            LIMIT 1
+        ");
+        $stmt->execute([$data['user_id'], $fingerprint]);
+        $exists = $stmt->fetch();
+
+        if ($exists) {
+            return (int)$exists['id'];
+        }
+    }
+
+    $billingYearMonth = get_transaction_billing_year_month(
+        (int)$data['user_id'],
+        (int)$data['account_id'],
+        (string)$data['transaction_date']
+    );
+
+    $stmt = $pdo->prepare("
+        INSERT INTO transactions
+        (
+            user_id,
+            account_id,
+            category_id,
+            transaction_type,
+            amount,
+            is_installment,
+            installment_months,
+            installment_interest_rate,
+            installment_interest_total,
+            installment_total_billed,
+            installment_prepay_amount,
+            transaction_date,
+            billing_year_month,
+            merchant_name,
+            description,
+            related_account_id,
+            source_loan_id,
+            fingerprint
+        )
+        VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?)
+    ");
+
+    $stmt->execute([
+        $data['user_id'],
+        $data['account_id'],
+        $data['category_id'],
+        $data['transaction_type'],
+        $data['amount'],
+        !empty($data['is_installment']) ? 1 : 0,
+        $data['installment_months'] ?? null,
+        $data['installment_interest_rate'] ?? 0,
+        $data['installment_interest_total'] ?? 0,
+        $data['installment_total_billed'] ?? null,
+        $data['installment_prepay_amount'] ?? 0,
+        $data['transaction_date'],
+        $billingYearMonth,
+        $data['merchant_name'] ?? null,
+        $data['description'] ?? null,
+        $data['related_account_id'] ?? null,
+        $sourceLoanId,
+        $fingerprint,
+    ]);
+
+    return (int)$pdo->lastInsertId();
+}
diff --git a/app/lib/transaction_service.php b/app/lib/transaction_service.php
index f58fdf0..93cfd32 100644
--- a/app/lib/transaction_service.php
+++ b/app/lib/transaction_service.php
@@ -1,56 +1,14 @@
 <?php
 
-require_once __DIR__ . '/db.php';
-require_once __DIR__ . '/account_service.php';
-require_once __DIR__ . '/installment_service.php';
-require_once __DIR__ . '/helpers.php';
-require_once __DIR__ . '/card_billing_service.php';
-
-function build_transaction_fingerprint(
-    int $userId,
-    int $accountId,
-    ?int $relatedAccountId,
-    int $categoryId,
-    string $transactionType,
-    float $amount,
-    string $transactionDate,
-    ?string $merchantName,
-    ?string $description
-): string {
-    $raw = implode('|', [
-        $userId,
-        $accountId,
-        $relatedAccountId ?? 0,
-        $categoryId,
-        $transactionType,
-        number_format($amount, 2, '.', ''),
-        $transactionDate,
-        trim((string)$merchantName),
-        trim((string)$description),
-    ]);
-
-    return hash('sha256', $raw);
-}
-
-function get_account_for_transaction(int $userId, int $accountId): ?array
-{
-    $pdo = db();
-
-    $stmt = $pdo->prepare("
-        SELECT *
-        FROM accounts
-        WHERE id = ?
-          AND user_id = ?
-        LIMIT 1
-    ");
-    $stmt->execute([$accountId, $userId]);
-
-    $row = $stmt->fetch();
-    return $row ?: null;
-}
-
-function create_transaction(array $data, bool $skipIfDuplicate = false): bool
-{
+require_once __DIR__ . '/db.php';
+require_once __DIR__ . '/account_service.php';
+require_once __DIR__ . '/installment_service.php';
+require_once __DIR__ . '/helpers.php';
+require_once __DIR__ . '/card_billing_service.php';
+require_once __DIR__ . '/transaction_record_service.php';
+
+function create_transaction(array $data, bool $skipIfDuplicate = false): bool
+{
     $pdo = db();
     $pdo->beginTransaction();
 
@@ -83,20 +41,7 @@ function create_transaction(array $data, bool $skipIfDuplicate = false): bool
             }
         }
 
-        $account = get_account_for_transaction(
-            (int)$data['user_id'],
-            (int)$data['account_id']
-        );
-
-        $billingYearMonth = null;
-        if ($account) {
-            $billingYearMonth = get_card_billing_year_month(
-                $account,
-                (string)$data['transaction_date']
-            );
-        }
-
-        $isInstallment = !empty($data['is_installment']) ? 1 : 0;
+        $isInstallment = !empty($data['is_installment']) ? 1 : 0;
         $installmentMonths = !empty($data['installment_months']) ? (int)$data['installment_months'] : null;
         $installmentInterestRate = !empty($data['installment_interest_rate']) ? (float)$data['installment_interest_rate'] : 0.0;
 
@@ -146,51 +91,14 @@ function create_transaction(array $data, bool $skipIfDuplicate = false): bool
             $installmentTotalBilled = null;
         }
 
-        $stmt = $pdo->prepare("
-            INSERT INTO transactions
-            (
-                user_id,
-                account_id,
-                category_id,
-                transaction_type,
-                amount,
-                is_installment,
-                installment_months,
-                installment_interest_rate,
-                installment_interest_total,
-                installment_total_billed,
-                installment_prepay_amount,
-                transaction_date,
-                billing_year_month,
-                merchant_name,
-                description,
-                related_account_id,
-                fingerprint
-            )
-            VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?, ?, 0, ?, ?, ?, ?, ?, ?)
-        ");
-
-        $stmt->execute([
-            $data['user_id'],
-            $data['account_id'],
-            $data['category_id'],
-            $data['transaction_type'],
-            $data['amount'],
-            $isInstallment,
-            $installmentMonths,
-            $installmentInterestRate,
-            $installmentInterestTotal,
-            $installmentTotalBilled,
-            $data['transaction_date'],
-            $billingYearMonth,
-            $data['merchant_name'],
-            $data['description'],
-            $data['related_account_id'],
-            $fingerprint,
-        ]);
-
-        $transactionId = (int)$pdo->lastInsertId();
-        $pdo->commit();
+        $transactionId = insert_transaction_record(array_merge($data, [
+            'is_installment' => $isInstallment,
+            'installment_months' => $installmentMonths,
+            'installment_interest_rate' => $installmentInterestRate,
+            'installment_interest_total' => $installmentInterestTotal,
+            'installment_total_billed' => $installmentTotalBilled,
+        ]), false);
+        $pdo->commit();
 
         if (
             $data['transaction_type'] === 'expense' &&
@@ -281,18 +189,11 @@ function update_transaction(int $transactionId, int $userId, array $data): void
             $data['description'] ?? null
         );
 
-        $account = get_account_for_transaction(
-            $userId,
-            (int)$data['account_id']
-        );
-
-        $billingYearMonth = null;
-        if ($account) {
-            $billingYearMonth = get_card_billing_year_month(
-                $account,
-                (string)$data['transaction_date']
-            );
-        }
+        $billingYearMonth = get_transaction_billing_year_month(
+            $userId,
+            (int)$data['account_id'],
+            (string)$data['transaction_date']
+        );
 
         $isInstallment = !empty($data['is_installment']) ? 1 : 0;
         $installmentMonths = !empty($data['installment_months']) ? (int)$data['installment_months'] : null;
@@ -438,4 +339,4 @@ function delete_transaction(int $transactionId, int $userId): void
         }
         throw $e;
     }
-}
\ No newline at end of file
+}
diff --git a/public/transaction_create.php b/public/transaction_create.php
index fe64d3e..7362582 100644
--- a/public/transaction_create.php
+++ b/public/transaction_create.php
@@ -118,73 +118,17 @@ if ($_SERVER['REQUEST_METHOD'] === 'POST') {
             'save_as_defaults' => !empty($_POST['save_as_defaults']) ? 1 : 0,
         ];
 
-        $transactionType = $form['transaction_type'];
-        $accountId = (int)$form['account_id'];
-        $categoryId = (int)$form['category_id'];
-        $amount = (float)str_replace(',', '', $form['amount']);
-        $transactionDate = $form['transaction_date'];
-        $merchantName = $form['merchant_name'];
-        $description = $form['description'];
-        $relatedAccountId = $form['related_account_id'] > 0 ? (int)$form['related_account_id'] : null;
-
-        $isInstallment = (int)$form['is_installment'];
-        $installmentMonths = $form['installment_months'] !== '' ? (int)$form['installment_months'] : null;
-        $installmentInterestRate = $form['installment_interest_rate'] !== '' ? (float)$form['installment_interest_rate'] : 0.0;
-        $installmentInterestTotal = $form['installment_interest_total'] !== ''
-            ? (float)str_replace(',', '', $form['installment_interest_total'])
-            : null;
-        $installmentTotalBilled = $form['installment_total_billed'] !== ''
-            ? (float)str_replace(',', '', $form['installment_total_billed'])
-            : null;
-
-        if ($amount <= 0) {
-            throw new RuntimeException('금액은 0보다 커야 합니다.');
-        }
-
-        if (!in_array($transactionType, ['income', 'expense', 'transfer', 'card_payment'], true)) {
-            throw new RuntimeException('거래 유형이 올바르지 않습니다.');
-        }
-
-        if ($accountId <= 0) {
-            throw new RuntimeException('주 계좌/카드를 선택하세요.');
-        }
-
-        if ($categoryId <= 0) {
-            throw new RuntimeException('카테고리를 선택하세요.');
-        }
-
-        if (in_array($transactionType, ['transfer', 'card_payment'], true) && !$relatedAccountId) {
-            throw new RuntimeException('관련 계좌를 선택해야 합니다.');
-        }
-
-        if ($relatedAccountId && $relatedAccountId === $accountId) {
-            throw new RuntimeException('주 계좌와 관련 계좌는 같을 수 없습니다.');
-        }
-
-        if ($transactionType !== 'expense') {
-            $isInstallment = 0;
-            $installmentMonths = null;
-            $installmentInterestRate = 0.0;
-            $installmentInterestTotal = null;
-            $installmentTotalBilled = null;
-        }
-
-        create_transaction([
-            'user_id' => $uid,
-            'account_id' => $accountId,
-            'category_id' => $categoryId,
-            'transaction_type' => $transactionType,
-            'amount' => $amount,
-            'transaction_date' => $transactionDate,
-            'merchant_name' => $merchantName !== '' ? $merchantName : null,
-            'description' => $description !== '' ? $description : null,
-            'related_account_id' => $relatedAccountId,
-            'is_installment' => $isInstallment,
-            'installment_months' => $installmentMonths,
-            'installment_interest_rate' => $installmentInterestRate,
-            'installment_interest_total' => $installmentInterestTotal,
-            'installment_total_billed' => $installmentTotalBilled,
-        ]);
+        $payload = build_transaction_payload_from_input($form, $uid);
+        $transactionType = $payload['transaction_type'];
+        $accountId = (int)$payload['account_id'];
+        $categoryId = (int)$payload['category_id'];
+        $merchantName = (string)($payload['merchant_name'] ?? '');
+        $relatedAccountId = $payload['related_account_id'] ? (int)$payload['related_account_id'] : null;
+        $isInstallment = (int)$payload['is_installment'];
+        $installmentMonths = $payload['installment_months'];
+        $installmentInterestRate = (float)$payload['installment_interest_rate'];
+
+        create_transaction($payload);
 
         if ($form['save_as_defaults']) {
             $save = [
@@ -877,4 +821,4 @@ require __DIR__ . '/../app/views/header.php';
 })();
 </script>
 
-<?php require __DIR__ . '/../app/views/footer.php'; ?>
\ No newline at end of file
+<?php require __DIR__ . '/../app/views/footer.php'; ?>
diff --git a/public/transaction_edit.php b/public/transaction_edit.php
index 2c7b081..a02bbcb 100644
--- a/public/transaction_edit.php
+++ b/public/transaction_edit.php
@@ -41,55 +41,8 @@ $categories = $stmt->fetchAll();
 
 if ($_SERVER['REQUEST_METHOD'] === 'POST') {
     try {
-        $transactionType = $_POST['transaction_type'] ?? '';
-        $accountId = (int)($_POST['account_id'] ?? 0);
-        $categoryId = (int)($_POST['category_id'] ?? 0);
-        $amount = (float)str_replace(',', '', (string)($_POST['amount'] ?? 0));
-        $transactionDate = $_POST['transaction_date'] ?? date('Y-m-d');
-        $merchantName = trim($_POST['merchant_name'] ?? '');
-        $description = trim($_POST['description'] ?? '');
-        $relatedAccountId = !empty($_POST['related_account_id']) ? (int)$_POST['related_account_id'] : null;
-
-        $isInstallment = (int)($_POST['is_installment'] ?? 0);
-        $installmentMonths = !empty($_POST['installment_months']) ? (int)$_POST['installment_months'] : null;
-        $installmentInterestRate = !empty($_POST['installment_interest_rate']) ? (float)$_POST['installment_interest_rate'] : 0.0;
-        $installmentInterestTotal = isset($_POST['installment_interest_total']) && $_POST['installment_interest_total'] !== ''
-            ? (float)str_replace(',', '', (string)$_POST['installment_interest_total'])
-            : null;
-        $installmentTotalBilled = isset($_POST['installment_total_billed']) && $_POST['installment_total_billed'] !== ''
-            ? (float)str_replace(',', '', (string)$_POST['installment_total_billed'])
-            : null;
-
-        if ($amount <= 0) throw new RuntimeException('금액은 0보다 커야 합니다.');
-        if (!in_array($transactionType, ['income', 'expense', 'transfer', 'card_payment'], true)) throw new RuntimeException('거래 유형이 올바르지 않습니다.');
-        if ($accountId <= 0) throw new RuntimeException('주 계좌/카드를 선택하세요.');
-        if ($categoryId <= 0) throw new RuntimeException('카테고리를 선택하세요.');
-        if (in_array($transactionType, ['transfer', 'card_payment'], true) && !$relatedAccountId) throw new RuntimeException('관련 계좌를 선택해야 합니다.');
-        if ($relatedAccountId && $relatedAccountId === $accountId) throw new RuntimeException('주 계좌와 관련 계좌는 같을 수 없습니다.');
-
-        if ($transactionType !== 'expense') {
-            $isInstallment = 0;
-            $installmentMonths = null;
-            $installmentInterestRate = 0.0;
-            $installmentInterestTotal = null;
-            $installmentTotalBilled = null;
-        }
-
-        update_transaction($id, $uid, [
-            'account_id' => $accountId,
-            'category_id' => $categoryId,
-            'transaction_type' => $transactionType,
-            'amount' => $amount,
-            'transaction_date' => $transactionDate,
-            'merchant_name' => $merchantName !== '' ? $merchantName : null,
-            'description' => $description !== '' ? $description : null,
-            'related_account_id' => $relatedAccountId,
-            'is_installment' => $isInstallment,
-            'installment_months' => $installmentMonths,
-            'installment_interest_rate' => $installmentInterestRate,
-            'installment_interest_total' => $installmentInterestTotal,
-            'installment_total_billed' => $installmentTotalBilled,
-        ]);
+        $payload = build_transaction_payload_from_input($_POST, $uid);
+        update_transaction($id, $uid, $payload);
 
         redirect('/transactions.php');
     } catch (Throwable $e) {