This is a most common requirement raised by many customers, please note the following points
a) A PO is not a negotiation document, its a document authorizing supply of goods at the agreed priced. Hence rationally there cannot be a discount on a PO
b) In case the buyer feels the necessity to showcase the savings achieved by him, he may use the "Market Price" field to enter the Pre-negotiated price of the item.
c) At the end of the quarter, you may run a report called "Savings by Buyer" which will highlight the cumulative discounts achieved by the buyer.
If still need be, create a DFF at header level called discount percentage, on entering a value, decrement all prices uniformly across items using a custom pl/sql procedure.