Wrong collection of taxes by builder

I bought one unit of 500sq.ft from WTC Noida during the month of April’ 17. I started paying the amount for the unit to the builder later and GST came into effect from 1st of July’ 17 and the percentage of GST is 12% whereas earlier tax rate before GST is 4.5%. In order to avoid the higher tax I pooled out all the money to the maximum extent and I paid to the builder before 30th of June’17.

I paid to the builder around 73% of the total value of the property plus taxes applicable at the rate of 4.5%. Now builder has adjusted 50% of the value of the money with 4.5% tax and he is demanding to pay for the balance amount 12% GST. I asked him why you are not adjusting the total amount that I paid before the date of GST implementation.
I am not getting any answer from him and he is asking me to pay 12% and he says extra money over and above 50% of the value is just advance only. by paying him 12% GST I will be loosing an extra amount of Rs. 47,593/- whom I have to complain and how I can be justified. Please help me out.

Ravi Kumar
Bengaluru 560100, Karnataka
Email: rxxxxxxxxu@hotmail.com