fix(pos): add item in the existing item row when discount is applied

(cherry picked from commit bee2c04d0b)

erpnext/selling/page/point_of_sale/pos_controller.js

@@ -682,7 +682,7 @@ erpnext.PointOfSale.Controller = class {
 					i.item_code === item_code &&
 					(!has_batch_no || (has_batch_no && i.batch_no === batch_no)) &&
 					i.uom === uom &&
-					i.rate === flt(rate)
+					i.price_list_rate === flt(rate)
 			);
 		}