Applying Discounts Value Against Commission Rather Off Vendor Share

Does anyone know a way to apply the discount value against the commission rather than the vendor share

If we do a 10% offer on say an order for £10 the supplier gets their share minus the discount (£1) and minus the commission of £2 (based on 20% commission )so they only get £7

We want to drive offers and promotions so we want the £1 to come out of our commission of £2 .

Is there an add on or fix to do this ?