Based on the above screen shot, you can observe credit group and check credit limit is assigned, this is the reason system is blocking the document when even customer exceeds credit limit,or if any open items are over due etc.
Since you have already mentioned that for your returns item category you have removed tick for the credit active field,this make sense because when you returning the goods system should not update the open order value in FD32.
Coming into your current process to by pass the credit block, there is no other alternative but approve the document through t code VKM1/ VKM3.
I don't know what reason your business process has designed a single sales document type for both standard sales and return sales.
But i have some following doubts on your process.
1. If you are using the same document type for both returns and sales orders there are so many dependence, example item category, scheduline category determination etc, all the time you need to change them manually when you are creating relevant process.
thanks,
Srinu.