Aumatic condition TANN when i use TAN

Hi,

I am trying to do the following process in SAP.

I want to make a condition in a specific customer that when he buy me 10 units i will give him 2 units. So i want to use condition TANN but i want that this condition should appear in the invoice automaticaly.

In advance i must say that I want to do this in a intercompany billing process so my invoice is based on a delivery and that delivery is based on a purchase order.

Best Regards

João Fernandes