diff --git a/backend/internal/repository/user_repo.go b/backend/internal/repository/user_repo.go
index 0d8c25c6..006a5464 100644
--- a/backend/internal/repository/user_repo.go
+++ b/backend/internal/repository/user_repo.go
@@ -329,17 +329,20 @@ func (r *userRepository) UpdateBalance(ctx context.Context, id int64, amount flo
 	return nil
 }
 
+// DeductBalance 扣除用户余额
+// 透支策略：允许余额变为负数，确保当前请求能够完成
+// 中间件会阻止余额 <= 0 的用户发起后续请求
 func (r *userRepository) DeductBalance(ctx context.Context, id int64, amount float64) error {
 	client := clientFromContext(ctx, r.client)
 	n, err := client.User.Update().
-		Where(dbuser.IDEQ(id), dbuser.BalanceGTE(amount)).
+		Where(dbuser.IDEQ(id)).
 		AddBalance(-amount).
 		Save(ctx)
 	if err != nil {
 		return err
 	}
 	if n == 0 {
-		return service.ErrInsufficientBalance
+		return service.ErrUserNotFound
 	}
 	return nil
 }
diff --git a/backend/internal/repository/user_repo_integration_test.go b/backend/internal/repository/user_repo_integration_test.go
index ab2195e3..19e6d6a3 100644
--- a/backend/internal/repository/user_repo_integration_test.go
+++ b/backend/internal/repository/user_repo_integration_test.go
@@ -290,9 +290,14 @@ func (s *UserRepoSuite) TestDeductBalance() {
 func (s *UserRepoSuite) TestDeductBalance_InsufficientFunds() {
 	user := s.mustCreateUser(&service.User{Email: "insuf@test.com", Balance: 5})
 
+	// 透支策略：允许扣除超过余额的金额
 	err := s.repo.DeductBalance(s.ctx, user.ID, 999)
-	s.Require().Error(err, "expected error for insufficient balance")
-	s.Require().ErrorIs(err, service.ErrInsufficientBalance)
+	s.Require().NoError(err, "DeductBalance should allow overdraft")
+
+	// 验证余额变为负数
+	got, err := s.repo.GetByID(s.ctx, user.ID)
+	s.Require().NoError(err)
+	s.Require().InDelta(-994.0, got.Balance, 1e-6, "Balance should be negative after overdraft")
 }
 
 func (s *UserRepoSuite) TestDeductBalance_ExactAmount() {
@@ -306,6 +311,19 @@ func (s *UserRepoSuite) TestDeductBalance_ExactAmount() {
 	s.Require().InDelta(0.0, got.Balance, 1e-6)
 }
 
+func (s *UserRepoSuite) TestDeductBalance_AllowsOverdraft() {
+	user := s.mustCreateUser(&service.User{Email: "overdraft@test.com", Balance: 5.0})
+
+	// 扣除超过余额的金额 - 应该成功
+	err := s.repo.DeductBalance(s.ctx, user.ID, 10.0)
+	s.Require().NoError(err, "DeductBalance should allow overdraft")
+
+	// 验证余额为负
+	got, err := s.repo.GetByID(s.ctx, user.ID)
+	s.Require().NoError(err)
+	s.Require().InDelta(-5.0, got.Balance, 1e-6, "Balance should be -5.0 after overdraft")
+}
+
 // --- Concurrency ---
 
 func (s *UserRepoSuite) TestUpdateConcurrency() {
@@ -511,6 +529,6 @@ func (s *UserRepoSuite) TestUpdateConcurrency_NotFound() {
 func (s *UserRepoSuite) TestDeductBalance_NotFound() {
 	err := s.repo.DeductBalance(s.ctx, 999999, 5)
 	s.Require().Error(err, "expected error for non-existent user")
-	// DeductBalance 在用户不存在时返回 ErrInsufficientBalance 因为 WHERE 条件不匹配
-	s.Require().ErrorIs(err, service.ErrInsufficientBalance)
+	// DeductBalance 在用户不存在时返回 ErrUserNotFound
+	s.Require().ErrorIs(err, service.ErrUserNotFound)
 }