Header discount gives same value discount for return documents

Hi,

We are using a header value discount which is a group discount. It distributes the discount across the items proportionately. We have an issue when we create a return order with reference to the original invoice.

To give an example. Customer places order of $100 for 10 cartons. We give discount of $10. Order value is $90. Customer returns 1 carton after invoicing. When return order is created with reference to the invoice, the system calculates the price of 1 carton which is $10 but the discount value is still $10 instead of $1. Please let me know where I have to check to distribute discount value proportionate to the value for return documents as well.