You are allowed to maintain the purchase price for Credit Note, Adjustment increase, and Transfer transactions only.
For example, issue a Credit Note to keep track of goods returned by the customer without knowing the past record of purchase amount, and none of the stock item values will be updated.
Go to Housekeeping, Run Setup, General Setup, Stock Valuation, select any costing method except Fixed Cost; tick CN under Type contribute to costing (stock valuation & cost calculation) section.
Issue a CN with 5 quantity & unit price of RM200 (Amount = RM1000).
Go to Transaction, Generate/Update, Maintain Purchase Price, tick Credit Note, click OK, key in 500 under Purc. Amount column.
Go to Reports, Inventory Reports, Item Status & Value, and the result will show the unit price of RM100 instead of RM200 from the previous record.