Sales unit- returns price

Hi Friends,

Looking for your suggestions in the following scenario.

We are mantaing sales price by sales unit. Example: 10 boxes in a carton. we sell materials to customers in cartons only, lets say each carton price of a particular material is rs.1000/-.

Customer returned one box, how to configure the system to consider one box price is 100/-, so that we can issue credit note to the customer what ever he returnd.

Awaiting for your suggestions.

Regards,

Vamsi P