Percent discount on POs

We have two scenarios where we want to calculate and apply a percent discount to a requisition and/or purchase order and are not sure how.

1) Percent discount off PO price

We have a PO with multiple lines, for a total of $4,000. When we receive the vendor invoice, it unexpectedly includes a 15% discount off each item, which reduces the total PO amount by $600, so we would owe $3,400 instead of $4,000.

We are not sure how to reflect this on the PO. While we could pay the full PO amount of $4,000 and deduct the $600 from the payment voucher (either in the Misc field or with a credit voucher), the PO itself would not show the discounted price. We need the PO to show the discounted price because a hard copy needs to be submitted for grant reimbursement.