Service tax on flat

I have booked a flat in Feb 2011 and the agreement was signed in March 2011. At that time the construction was almost complete ( except plaster, flooring, pint and electrical).
I paid him 20% amount before agreement and remaining was housing loan which was sanctioned after agreement.
The builder gave me only one demand letter in April 2012 stating that the said flat is 100% complete and thus pay the balance 80% amount and this letter was given to bank for disbursement.
He handed over the flat to me without possession letter in July 2011 and i started living there.
Later after 6-7 months he  demanded VAT and service tax which he had not intimated before.
I paid him the VAT and he is demanding for service tax (almost 3.08%).
Someone told me that buyer is not liable to pay service tax if the flat is purchase after completion. He also told me that if I have booked a flat in midway when some part of the flat is completed, then i am liable to pay service tax only on the amount of remaining work of the flat.
In my case, at the time of booking, the flat was almost 75% complete. The only documentary evidence is the demand letter which he gave me after 1.5 months from booking stating that the said flat is 100% complete.

Please tell me how much service tax should i pay. The builder is demanding for service tax on the whole amount.

Please reply.
Thanks and regards.
Prof. Rahul Padalkar