Discount at the PO LEVEL ???

Hi,

HOPE YOU ALL OK ..

Is there any provision on the Purchasing Module of oracle , on the basis of which we can calculated the discount on the Purchase Order.
let say supplier offers us a specific item at the rate of 20$  per unit but he also said if we purchase 20 items on a instance so he offer 10% discount on total amount. So how to calculate that on the purchase order so that it can show the discount on the PO ??