Retrospective discounts for purchase orders

One of my customers receives a retrospective 10% purchase discount on some stock items from the supplier. They would like to be able to create purchase orders for these items at list price, but have them booked into stock at the discounted price, with the expected discount posted to a separate NL account. This is in fact the reverse of landed costs. For the moment, I have amended the system to allow a negative landed costs percentage, which works fine. But it would be fairly simple to change Sage 200 to include this feature.