Please refer the Rule 16 of CER 2002.
The sales returned goods needs to be kept seperately and have to intimate to local excise office immediately after receipt.
IF the officer doesnt react within 24hr from the date of intimation, you have to take the goods for further processing.
You can avail the CEnvat credit of these goods which has to be accounted in Form V register.
If you have take the Cenvat Credit and the process which you are going to carried out doesnt amount to manufacture then the quantum of cenvat credit needs to be pass which was taken at the time of receipt
If the process carried out amounts to manufacture, then the duty needs to paid on transaction value.
Hope this clears your doubt.