It may be that stock was not purchased via Jim2, and that it is not required to track the movement of this stock in Jim2.
The best way to deal with this is to create a stock code and ensure the type is Journal.
You can then raise a normal purchase order to the vendor with a negative amount and finish the as usual.
This will generate a credit on the creditor's account.
Further information
Add an RTV Against a Different Vendor
Add an RTV Against Multiple POs
Add Discount or Restocking Fee on a Return to Vendor
Add Stock to a Return to Vendor