What excise rate to be taken on sales return????

Hello friends & members ,

               We have sold goods on 27.02.12 to X party on this day excise rate was prevalling 10%, after that said party rejected the said goods & returned us by invoicing @ 12% excise rate on 29.08.012.

Query is that  they did right or wrong, means what rate they need to mention in invoice 10% OR 12%?????pls advice with necessary rules & section.................

Thanks & regards- Lokesh Pokharna