You can apply rule 16 of C. Excise Rules to sale returns. The rule permits you credit in the same manner in which credit is taken on inputs. However, instead of taking entry in DSA (RG-1), one should maintain a separate register under rule 16. The register should contain details such as date of receipt, quantity received, process carried out, and dispatch details.
The goods can be processed after receipt. Subsequent removal has to be under invoice. Excise liability upon removal will depend upon nature of the process undertaken. If the process amounts to manufacture one should pay duty at the current rate and current value. If the process does not amount to manufacture, one has to pay an amount equal to the credit taken.