Purchasing Module- applying Discounts for early payment

SUGGESTED

When entering the invoice in Purchasing against the PO I always change the dates and terms and apply the discount if paid early. However when posting to AP the discount does not carry over on the invoce and our AP person has to go and enter it manually. The Vendor is set up in AP with the discount terms so why does Purchasing to retain the discount and post it in to Accounts Payable?