GST Dispute With Builder

I have entered in a builder buyer agreement in September 2018 to buy Flat from Godrej Properties. Out of 10 equal installments, I have paid 3 timely installments so far and the GST on same has been charged at 12% but has not passed any input tax credit benefit to me. After notification from the government for changing of GST rate of 5% for under-construction houses, I asked my RM in Godrej properties to charge 5% GST for remaining installments. The RM says that since the project was launched under 12% GST regime, they cannot switch to 5% regime, as they will lose the benefit of input tax credit under 5% scheme. However, at the time of deciding the costing of the price of the flat, the costing for them was taken at 12%, so they will continue to charge GST at 12% Kindly advice if I have any legal recourse for getting the benefit from new GST rate of 5%