Service tax provision on manpower contractor 's bill

Dear Sir,

We are availing the services of man power supplier contractors and they have provided us their provisional bill for bonus related to F/Y 2012-13, in Mar.'13. We have provided for bonus for the purpose of Balance Sheet, calculated service tax on 75% of bonus under RCM and paid the same in the month of Mar.'13.

Now the contractors have raised the actual bills in the month of Sep'13, resulting in substantial reduction in the bonus liability as well as the service tax amount, that was payable by us.

How can we adjust or recover the excess service tax paid by us under RCM.

CA. Krishna Srivastava