Service tax credit on cancelled invoice

We have a client that has not accounted bills raised last year. One bill is dated June 2011 and the rest are between January and March 2012. The total amount is about Rs 5 lakhs. They are ready to pay now if we raise fresh bills for the amount.

Let me know if that is possible and how I can make the adjustments for the service tax that we have already paid.

Regards

Lakshmi Devanand