diff --git a/backend/app/services/business_service.py b/backend/app/services/business_service.py
index 85ca8d4..00b0f0d 100644
--- a/backend/app/services/business_service.py
+++ b/backend/app/services/business_service.py
@@ -16,7 +16,7 @@
 
 import logging
 from datetime import date
-from decimal import Decimal
+from decimal import ROUND_HALF_UP, Decimal
 from fractions import Fraction
 
 from sqlalchemy import Row, func, select
@@ -136,12 +136,19 @@ def _build_item_rows(business_id: int, items: list[BusinessItemIn]) -> list[Busi
 
     不信任前端传值（schema 亦无 sale_amount 入参）——单价可空时销售额记 0，
     由用户在编辑中补价后重算（毛利润视图中销售额 0 的行全部为 0，防除零）。
+    乘积显式 quantize 两位固化：数量(2位)×单价(4位) 可产 6 位小数，直写
+    DECIMAL(14,2) 会被 MySQL 静默舍入且回显漂移（创建响应读内存值、复查
+    读库值前后不一致）；HALF_UP 与 MySQL 的 DECIMAL 舍入方向一致。
     """
     rows: list[BusinessItem] = []
     for item in items:
         quantity = Decimal(item.quantity)
         sale_price = Decimal(item.sale_price) if item.sale_price is not None else None
-        sale_amount = (quantity * sale_price) if sale_price is not None else Decimal("0")
+        sale_amount = (
+            (quantity * sale_price).quantize(Decimal("0.01"), rounding=ROUND_HALF_UP)
+            if sale_price is not None
+            else Decimal("0.00")
+        )
         rows.append(
             BusinessItem(
                 business_id=business_id,
diff --git a/backend/app/services/formula.py b/backend/app/services/formula.py
index 56c9b43..19d4f90 100644
--- a/backend/app/services/formula.py
+++ b/backend/app/services/formula.py
@@ -40,7 +40,12 @@ def salesperson_rate(
     start/floor 为 ‰ 数、slope 为 ‰/天（存储口径，slope 为精确分数 4/360）。
     ≤分界天数按「起始 − 天数×斜率」线性递减；超过分界天数固定取下限
     （对照表 #4：360 天=6‰，361 天起固定 4‰）。
+
+    先款后货的回款（payment_days 为负）钳制为 0 天：负天数线性外插会算出
+    高于起始值的比例（多算绩效），钳到 0 天即按起始比例计提；payment_days
+    的出参仍照实输出负值（回款登记照常入库，录入异常由台账留痕暴露）。
     """
+    days = max(days, 0)
     if days > boundary_days:
         return floor / 1000
     return (start - days * slope) / 1000
diff --git a/backend/app/services/shipment_service.py b/backend/app/services/shipment_service.py
index 1ba69f7..5b5c131 100644
--- a/backend/app/services/shipment_service.py
+++ b/backend/app/services/shipment_service.py
@@ -23,7 +23,7 @@
 """
 
 import logging
-from decimal import Decimal
+from decimal import ROUND_HALF_UP, Decimal
 
 from sqlalchemy.exc import IntegrityError
 from sqlalchemy.orm import Session
@@ -99,12 +99,19 @@ def _build_item_rows(items: list[ShipmentItemIn]) -> list[ShipmentItem]:
 
     不信任前端传值（schema 亦无 amount 入参）——单价可空时行金额记 0，
     由用户在编辑中补价后重算；shipment_id 由 replace_items 统一回填。
+    乘积显式 quantize 两位固化（模式同 business_service._build_item_rows）：
+    数量(2位)×单价(4位) 可产 6 位小数，直写 DECIMAL(14,2) 会被 MySQL
+    静默舍入且回显漂移。
     """
     rows: list[ShipmentItem] = []
     for item in items:
         quantity = Decimal(item.quantity)
         sale_price = Decimal(item.sale_price) if item.sale_price is not None else None
-        amount = (quantity * sale_price) if sale_price is not None else Decimal("0")
+        amount = (
+            (quantity * sale_price).quantize(Decimal("0.01"), rounding=ROUND_HALF_UP)
+            if sale_price is not None
+            else Decimal("0.00")
+        )
         rows.append(
             ShipmentItem(
                 product_id=item.product_id,
diff --git a/backend/tests/integration/test_businesses.py b/backend/tests/integration/test_businesses.py
index c37328c..c204048 100644
--- a/backend/tests/integration/test_businesses.py
+++ b/backend/tests/integration/test_businesses.py
@@ -225,6 +225,31 @@ class TestCreate:
         )
         assert resp.status_code == 400, resp.text
 
+    def test_行金额乘积显式两位固化_回显与库值一致(self, client: TestClient, normal: dict) -> None:
+        """数量(2位)×单价(4位) 可产 6 位小数：显式 HALF_UP 固化，与 MySQL 的
+        DECIMAL(14,2) 舍入方向一致 —— 直写不 quantize 会被库端静默舍入，
+        创建响应（读内存值）与复查（读库值）前后显示不一致。"""
+        headers = normal["headers"]
+        customer = _customer(client, headers)
+        data = _create(
+            client,
+            headers,
+            customer["id"],
+            items=[_item(quantity="1.55", sale_price="3.3333", cost_amount=None)],
+        )
+        # 1.55 × 3.3333 = 5.166615 → 5.17（HALF_UP；MySQL 同值同向）
+        item = client.get(f"{_BUSINESSES}/{data['id']}", headers=headers).json()["data"]["items"][0]
+        assert item["sale_amount"] == "5.17"
+        assert Decimal(data["total_sale_amount"]) == Decimal("5.17")
+
+        # 库值与回显一致（无「内存 5.166615 / 库里 5.17」的漂移）
+        factory = get_session_factory()
+        with factory() as session:
+            db_amount = session.execute(
+                select(BusinessItem.sale_amount).where(BusinessItem.business_id == data["id"])
+            ).scalar_one()
+        assert db_amount == Decimal("5.17")
+
     def test_明细行数量为0_400(self, client: TestClient, normal: dict) -> None:
         customer = _customer(client, normal["headers"])
         resp = client.post(
diff --git a/backend/tests/unit/test_formula.py b/backend/tests/unit/test_formula.py
index f252d17..7a85a48 100644
--- a/backend/tests/unit/test_formula.py
+++ b/backend/tests/unit/test_formula.py
@@ -69,6 +69,14 @@ class TestSalespersonRate:
         assert _sp_rate(360) == Fraction(6, 1000)
         assert _sp_rate(360) != Fraction(6)
 
+    def test_负天数钳制为0天_按起始比例计提(self) -> None:
+        """先款后货（payment_days 为负）不线性外插：钳到 0 天取起始比例。
+
+        修复前 days=-5 会算出 (10 + 5×4/360)‰ —— 高于起始值，多算绩效。
+        """
+        assert _sp_rate(-5) == _sp_rate(0)
+        assert _sp_rate(-1) == Fraction(10, 1000)
+
 
 class TestProfitRate:
     """毛利润率：不含运费口径（对照表 #2），毛利润额 / 销售额。"""