The user can adjust the value of the product in Stock Locations, please check out this

article, How do I change the item value in my Stock Location?

However, the value of the product in the Purchase Order can't be changed to match the value of the product in an Invoice.

Similarly, the total value of the Purchase Order can't be changed to match the value on the Invoice. Once the Purchase Requisition has been approved and converts to a Purchase Order, all prices are final.