If a supplier includes a discount on your quote, this does not change the Maximum Amount of your BPO. The BPO must be set up for the pre-discount amount. Make sure to set the Maximum for the line item and the Maximim Limit on the Summary of the BPO for the pre-discounted (full price) amount.
The supplier must include the discount amount on the invoice as part of the Header Charges when the invoice is submitted. The header charge area is used for discounts, tax, shipping and handling. When receiving, you must receive for the full amount of the line item price as well. The header discount will be applied and the payment will be issued for the amount minus the discount.