Ability to add a discount to Purchase Orders

For significant projects, we can often get a disccount from a supplier for goods.

This is often applied as a bulk discount on either the whole purchase order, or on specific goods.

There is currently no way for us to reflect this on the purchase order, short of recalculating the item cost of every single item, which is quite frustrating.
We don't want to leave it at regular price, as (sometimes) the manufacturer forgets to apply the discount and we need to call up to get it amended when it doesn't match the PO.

Having a discount column per item where we can enter an amount or a percentage would be awesome.