It is just because of your client is not willing to carry out one additional work. But you can ask your client - at the time of returns, unless there is some trigger point, how system would know that there is a returns. For that, instead of using the original sale order, you should have a separate document type which is the Best practice. I am sure, this scenario would occur once in a while and for that if you deviate the standard SAP functionality, it will lead to many other complications.
G. Lakshmipathi