applicability of service tax

AT THE TIME OF PAYMENT OF ADVANCE TO THE PARTY FOR PROVIDING SOME SERVICE THE RATE OF SERVICE TAX WAS 10 %. BUT AT THE TIME OF PROVIDING THE SERVICE THE RATE OF TAX WAS 12%. SO AT WHICH RATE SERVICE TAX WAS PAYABLE TO THE PARTY?

SINCE THE PRICE WAS INCLUSIVE OF SERVICE TAX, THE PARTY ALREADY DEPOSITTHE SERVICE TAX ON ADVANCE RECEIVED @ 10%.