Excise duty - cvd & sad adjustments

Hi All,

We have sold imported materials to one of our interstate client in Sep 2015 and transferred to them CVD 12.5% and SAD 4%. We have filed Dealer/Importer return for CVD and SAD transfer online in Oct 2015. But now the client return the part of some material in Dec 2015. 
Please clarify, whether we have to file revised Dealer/Importer return or we have to adjust the amount of CVD and SAD in their next billing transfer.