Delay in possession after paying all the amount mentioned in agreement

I booked a flat on 02 oct 2016 and my loan got approved in Feb 2017. After withdrawing two installments form bank builder has given me third demand letter on 09 Jun 2017 and seeking payment before 24 Jun 2017. Being an NRI customer builder has promised me that he will get the payment from the bank if we provide them signed disbursement letter. For June demand he did not come to us for signature and again he sends the same demand in July and added GST in the new demand. This added GST they never asked till the final demand. At the time of registration, I asked them for the total outstanding amount and whatever amount they mentioned as the pending amount, I paid all, even at the time of final payment they sent me a mail asking for pending amount and accordingly I paid whatever they asked. After the final payment which was made on 14 Jun 2018, when I asked for possession, they started saying we have approx Rs. 25k is due. Then I asked form they this due came they sad you did not pay GST but I sad I was regularly paying the Service tax and GST. Till date, They did not give me the possession. Can you suggest what should I do in this case? Although It was the fault from builder side that they did not go to the bank for money in the June months and for the same demand they went in July month but for this demand, they are charging extra 25k GST to me instead of service tax.