fix: keep source rate on re-fetch when maintain same rate is enabled

With "maintain same rate" on, re-fetching item details on a row mapped from a
source document (e.g. a Purchase Order) pulled the latest Item Price, giving a
rate the document can never be saved with. Skip the price list fetch for such
rows and keep the source rate.

Fixes frappe/erpnext#57436

erpnext/stock/get_item_details.py

@@ -40,6 +40,17 @@ purchase_doctypes = [
 
 NOT_APPLICABLE_TAX = "N/A"
 
+# Item rows mapped from these source documents must keep the source rate when
+# "maintain same rate" is enabled, so a newer Item Price is never fetched onto them.
+maintain_same_rate_source_fields = {
+	"Purchase Order": ("supplier_quotation_item",),
+	"Purchase Receipt": ("purchase_order_item",),
+	"Purchase Invoice": ("po_detail", "pr_detail"),
+	"Sales Order": ("quotation_item", "prevdoc_docname"),
+	"Delivery Note": ("so_detail", "si_detail"),
+	"Sales Invoice": ("so_detail", "dn_detail"),
+}
+
 
 def _preprocess_ctx(ctx):
 	if not ctx.price_list:
@@ -120,7 +131,11 @@ def get_item_details(
 	if ctx.doctype in ["Purchase Order", "Purchase Receipt", "Purchase Invoice"]:
 		ctx.customer = None
 
-	out.update(get_price_list_rate(ctx, item))
+	source_row = get_rate_locked_source_row(ctx, doc)
+	if source_row:
+		out.price_list_rate = flt(source_row.get("price_list_rate")) or flt(source_row.get("rate"))
+	else:
+		out.update(get_price_list_rate(ctx, item))
 
 	if (
 		not out.price_list_rate
@@ -188,6 +203,34 @@ def remove_standard_fields(out: frappe._dict):
 	return out
 
 
+def get_rate_locked_source_row(ctx: ItemDetailsCtx, doc) -> frappe._dict | None:
+	"""Return the current item row when its rate is locked to a source document.
+
+	With "maintain same rate" enabled, a row mapped from a source (e.g. a Purchase
+	Order) can only be saved at the source rate. Fetching a newer Item Price here
+	would set a rate the document can never be saved with, so keep the source rate.
+	"""
+	source_fields = maintain_same_rate_source_fields.get(ctx.doctype)
+	if not source_fields or not doc or ctx.get("is_return") or not maintain_same_rate_enabled(ctx):
+		return None
+
+	row = next((d for d in doc.get("items") or [] if d.get("name") == ctx.child_docname), None)
+	if row and any(row.get(field) for field in source_fields):
+		return row
+	return None
+
+
+def maintain_same_rate_enabled(ctx: ItemDetailsCtx) -> bool:
+	if ctx.doctype in purchase_doctypes:
+		if ctx.get("is_internal_supplier"):
+			return False
+		return bool(cint(frappe.get_cached_value("Buying Settings", "None", "maintain_same_rate")))
+
+	if ctx.get("is_internal_customer"):
+		return False
+	return bool(cint(frappe.get_cached_value("Selling Settings", "None", "maintain_same_sales_rate")))
+
+
 def set_valuation_rate(out: frappe._dict, ctx: frappe._dict):
 	from erpnext.selling.doctype.product_bundle.product_bundle import get_active_product_bundle
 

erpnext/stock/tests/test_get_item_details.py

@@ -124,3 +124,75 @@ class TestGetItemDetail(ERPNextTestSuite):
 		dn.save()
 		self.assertEqual(dn.items[0].batch_no, "BATCH01")
 		self.assertEqual(dn.items[0].rate, 50)
+
+	def test_maintain_same_rate_keeps_source_rate_on_refetch(self):
+		"""#57436: with "maintain same rate" on, re-fetching a PR row mapped from a
+		PO must keep the PO rate instead of pulling a newer, higher Item Price.
+
+		The rate is validated on save, so it can never persist changed; assert the
+		fetched rate directly to prove the newer Item Price is never picked up.
+		"""
+		from erpnext.buying.doctype.purchase_order.mapper import make_purchase_receipt
+		from erpnext.buying.doctype.purchase_order.test_purchase_order import create_purchase_order
+		from erpnext.stock.doctype.item.test_item import make_item
+
+		def set_maintain_same_rate(value):
+			frappe.db.set_single_value("Buying Settings", "maintain_same_rate", value)
+			frappe.clear_cache(doctype="Buying Settings")
+
+		set_maintain_same_rate(1)
+
+		item_code = make_item(properties={"is_stock_item": 1}).name
+		po = create_purchase_order(item_code=item_code, qty=1, rate=100)
+
+		# The PO may auto-insert an Item Price at 100; bump it to the newer, higher rate.
+		item_price = frappe.db.get_value(
+			"Item Price", {"item_code": item_code, "price_list": "Standard Buying"}
+		)
+		if item_price:
+			frappe.db.set_value("Item Price", item_price, "price_list_rate", 120)
+		else:
+			frappe.get_doc(
+				{
+					"doctype": "Item Price",
+					"price_list": "Standard Buying",
+					"item_code": item_code,
+					"price_list_rate": 120,
+				}
+			).insert()
+
+		pr = make_purchase_receipt(po.name)
+		pr.insert()
+
+		def fetch_price_list_rate():
+			ctx = frappe._dict(
+				{
+					"item_code": item_code,
+					"doctype": "Purchase Receipt",
+					"name": pr.name,
+					"company": pr.company,
+					"supplier": pr.supplier,
+					"currency": pr.currency,
+					"conversion_rate": 1.0,
+					"price_list": "Standard Buying",
+					"price_list_currency": pr.currency,
+					"plc_conversion_rate": 1.0,
+					"warehouse": pr.items[0].warehouse,
+					"uom": pr.items[0].uom,
+					"stock_uom": pr.items[0].stock_uom,
+					"qty": pr.items[0].qty,
+					"child_doctype": pr.items[0].doctype,
+					"child_docname": pr.items[0].name,
+					"is_return": 0,
+					"is_internal_supplier": 0,
+					"ignore_pricing_rule": 1,
+				}
+			)
+			return get_item_details(ctx, pr).get("price_list_rate")
+
+		# Rate stays at the PO rate; the newer Item Price (120) is not fetched.
+		self.assertEqual(fetch_price_list_rate(), 100)
+
+		# Control: without the setting the newer Item Price would be fetched.
+		set_maintain_same_rate(0)
+		self.assertEqual(fetch_price_list_rate(), 120)