Property purchase service tax

Hello, This is regarding property service tax , Booking Date :- 28-Mar-11 Booking amount of 100000/- is debited from my account on 30-Mar-11 Builder Completion Certificate Date :- 31-Mar-11 Sale Agreement Date :- 11-Apr-11 Payment made after completion certificate :- Appro. 31 Lakhs. Please confirm in above scenario do I need to pay service tax on whole agreement cost or service tax would be on payment made before getting completion certificate(i.e Rs.100000/-). Please clarify the above doubt. Thanks in advance !! ~Dinesh