Tnvat on buying new flat

Hi.

Can anyone help me out with the following problem ASAP?

We have purchased a flat in the year 2014 and it is nearing possesion by 2 weeks. Suddenly the builder is saying that there is an additional TNVAT of rs50000. Do we need to pay TNVAT for purchasing a flat? we have already paid stamp duty at the time of registration. Also, the rate per sq. ft. including construction was fixed at rs2600. 

Pls help me soon.