Discount process in returns in consignment .

Dear sd Experts

i have one business process that company is giving some discount to Dealer In consignment  process

ex: Company sent to dealer 100 Bikes and he sold bikes to end customer

so company wants to give discount 100000 rs , for each bike 1000 rs

i have configured This , 089 condition table  and created condition record for CI ( KE Document type ) we activated it means system will give the discount to Dealer..

This is absolutely Fine working But

Incase end customer sends stock back to Dealer then that discount should be deducted

ex: Dealer sent stock 100 bikes and dealer  sent stock to end customer 100 bikes ... now automatically Dealer got discount 100000 rs

Now imagine end customer sent stock back 5 bikes ...This is consignment returns he said i dont want this stock .

so in this case My company wants me to reduce discount automatically .. like 100000-5000: 95000 rs should be there

Beceasue Generally that return discount is not added

SO How to achieve this