Credits on Purchase Order?

Good afternoon,

I have a situation in which a supplier wants to give us a substancial credit for placing a first time order with them. The supplier has asked that the credited amount be part of the PO. When our users try to enter a negative amoutn on the PO, they get the error message "Price must be a non-negative value". This makes sense to me; however, has anyone else out there encountered such an issue? How have you handled it? We usually handle our credits through invoicing and not in the PO. Your suggestions will be greatly approciated.

Thank you!