refactor: 잘못된 로직 추가

Backend/src/main/java/com/luckyseven/backend/domain/budget/entity/Budget.java

@@ -1,5 +1,6 @@
 package com.luckyseven.backend.domain.budget.entity;
 
+import com.luckyseven.backend.domain.budget.dto.BudgetUpdateRequest;
 import com.luckyseven.backend.domain.team.entity.Team;
 import com.luckyseven.backend.sharedkernel.entity.BaseEntity;
 import jakarta.persistence.Column;
@@ -66,7 +67,13 @@ public Budget(Team team, BigDecimal totalAmount, Long setBy,
 
   public void setTotalAmount(BigDecimal totalAmount) {
     this.totalAmount = totalAmount;
-    this.balance = totalAmount;
+  }
+
+  public void addBalance(BudgetUpdateRequest request) {
+    if (request.additionalBudget() == null) {
+      return;
+    }
+    this.balance = this.balance.add(request.additionalBudget());
   }
 
   public void setExchangeInfo(boolean isExchanged, BigDecimal amount, BigDecimal exchangeRate) {
@@ -77,29 +84,30 @@ public void setExchangeInfo(boolean isExchanged, BigDecimal amount, BigDecimal e
     }
 
     updateForeignBalance(amount, exchangeRate);
-    this.avgExchangeRate = exchangeRate;
-
   }
 
   public void updateExchangeInfo(boolean isExchanged, BigDecimal amount, BigDecimal exchangeRate) {
     if (!isExchanged) {
       return;
     }
 
-    updateForeignBalance(amount, exchangeRate);
     updateAvgExchangeRate(amount, exchangeRate);
+    updateForeignBalance(amount, exchangeRate);
 
   }
 
   // 예산 추가 후 외화잔고 및 평균환율 수정
   private void updateAvgExchangeRate(BigDecimal amount, BigDecimal exchangeRate) {
-    if (this.avgExchangeRate == null) {
+    if (this.avgExchangeRate == null || this.avgExchangeRate.compareTo(BigDecimal.ZERO) == 0) {
       avgExchangeRate = exchangeRate;
       return;
     }
-    this.avgExchangeRate = (this.balance.multiply(this.avgExchangeRate)
-        .add(amount.multiply(exchangeRate)))
-        .divide(this.balance.add(amount), 2, RoundingMode.HALF_UP);
+    BigDecimal foreignAmount = amount.divide(exchangeRate, 10,
+        RoundingMode.HALF_UP); // 외화 환산, 충분한 정밀도 확보
+    BigDecimal totalCost = this.foreignBalance.multiply(this.avgExchangeRate).add(amount);
+    BigDecimal totalForeign = this.foreignBalance.add(foreignAmount);
+    this.avgExchangeRate = totalCost.divide(totalForeign, 2, RoundingMode.HALF_UP);
+
   }
 
   private void updateForeignBalance(BigDecimal amount, BigDecimal exchangeRate) {
@@ -131,6 +139,7 @@ public Budget setTeam(Team team) {
 
     return this;
   }
+
   public void updateBalance(BigDecimal balance) {
     if (balance != null) {
       this.balance = balance;

Backend/src/main/java/com/luckyseven/backend/domain/budget/service/BudgetService.java

@@ -85,6 +85,7 @@ private static void addBudget(BudgetUpdateRequest request, Budget budget) {
           request.additionalBudget(),
           request.exchangeRate());
       budget.setTotalAmount(budget.getTotalAmount().add(request.additionalBudget()));
+      budget.addBalance(request);
     }
   }
 

Backend/src/main/java/com/luckyseven/backend/domain/expense/service/ExpenseService.java

@@ -11,6 +11,7 @@
 import com.luckyseven.backend.domain.expense.dto.ExpenseResponse;
 import com.luckyseven.backend.domain.expense.dto.ExpenseUpdateRequest;
 import com.luckyseven.backend.domain.expense.entity.Expense;
+import com.luckyseven.backend.domain.expense.enums.PaymentMethod;
 import com.luckyseven.backend.domain.expense.mapper.ExpenseMapper;
 import com.luckyseven.backend.domain.expense.repository.ExpenseRepository;
 import com.luckyseven.backend.domain.member.entity.Member;
@@ -46,13 +47,24 @@ public CreateExpenseResponse saveExpense(Long teamId, ExpenseRequest request) {
     Member payer = findPayerOrThrow(request.payerId());
 
     Budget budget = team.getBudget();
-    validateSufficientBudget(request.amount(), budget.getBalance());
+
+    if (request.paymentMethod() == PaymentMethod.CASH) {
+      BigDecimal foreignAmount = request.amount();
+      BigDecimal KRWAmount = foreignAmount.multiply(budget.getAvgExchangeRate());
+      validateSufficientBudget(KRWAmount, budget.getBalance());
+      validateSufficientBudget(foreignAmount, budget.getForeignBalance());
+      budget.updateBalance(budget.getBalance().subtract(KRWAmount));
+      budget.setForeignBalance(budget.getForeignBalance().subtract(foreignAmount));
+
+    } else if (request.paymentMethod() == PaymentMethod.CARD) {
+      validateSufficientBudget(request.amount(), budget.getBalance());
+      budget.updateBalance(budget.getBalance().subtract(request.amount()));
+    }
 
     Expense expense = ExpenseMapper.fromExpenseRequest(request, team, payer);
     Expense saved = expenseRepository.save(expense);
 
     // TODO: 낙관적 락(Lock) 적용 검토
-    budget.updateBalance(budget.getBalance().subtract(request.amount()));
 
     createAllSettlements(request, payer, saved);