Need to apply straight 5% discount to PO Voucher not based on terms

How can I apply a 5% discount across a PO Voucher with multiple lines not based on payment terms?

For example, receive an invoice based on a PO with 2 lines

Line 1 - Qty 1 @ $10

Line 2 - Qty 5 @ $5

The vendor is giving an overall discount of 5% which equals $1.75 regardless of when the invoice is paid.

What is the best method of applying this discount so each line gets credited appropriately? Will the discounted amount go back on the PO?

We are also using Commitment Control, so how will the discount affect the encumbrance?

Thanks,
Alicia