Purchase Order Total Discount on Amount

Where to add total discount amount for the PO?

Situation:

I have PO $100.50 but need to discount $0.50. Where to add $0.50 discount in AX2012?

I only saw discount on % but no discount on amount. Any suggestion or work around?

Please help. Thanks alot!!

**I'm new in AX2012..please elaborate more in details in your answer