Hi Mark, I will just add a process comment to Gil's more technically elegant response.
There is a lot of functionality, and a lot of process options within the RMA functionality.
From a process perspective, the Disposition Codes are intended to envision the possible actions with regard to the Return. In the case of scrap, the idea is that you won't know it needs to be scrapped, until it arrives back. Once it is back in, and there is an inspection, the scrap transaction can happen at that point. So the act of scrapping is separate from the return. Two different transactions (you can add a comment to a Scrap transaction).
In your case, do you KNOW that you are going to scrap before it arrives back? Are you going to "pre-scrap"? If so, at that time to you KNOW the customer will be buying the replacement? Do they buy it as part of the Return? That purchase could be added to the Return.
See what I mean? Lots of options. Knowing the process or intended process would help with recommending an approach to solve the challenge. At this point not convinced that a modification is the solution.