How do you take a supplier special line item discount from a invoice?

Need some help.  We use 3 way match, and everytime a supplier gives us a line item special discount pricing on the invoice is there a process were we can take the discount and it update the PO.  For example we order an item for $10.00 with a qty of 3.  We received 3.  However, when we received the invoice the supplier has offered a special pricing for $5. 00.  How can we systemicatically take the discount and the PO be closed after payment.