GST effect of already Booked flat in PUNE PMRDA

Hello sir/mam

I have booked and registered under construction flat in PUNE keshvnagar I paid stamp duty and all my margins and 70% bank loan amount is already disbursed end of march2017 now remain amount is 625000/- which is to be disbursed.now builder is asking addition 7.5% on remaining amount that is Rs.46875/- is it correct or I am getting cheated