How to account for Purchase order discount in Invoice

Summary:

We have DFF's in Purchase order to capture Allowances and Discounts. When the Vendor sends the Invoice back, they sometimes do not apply the Invoice. How do we ensure that we avail the Discount even if the Invoice does not contain the Discount line?

Sometimes, the Invoice is completely missing the Discount information; sometimes, the vendor includes the Discount in the Invoice price (results in the Invoice going on hold). How do we capture and avail the discounts?

Content (required):

Scenario 1: PO has a discount as DFF. The invoice is sent without any discount, and the Vendor expects us to take the discount and pay.