Reduced GST rate on under construction flats

I booked an apartment in one of the Shriram Projects and paid RS.90000 as booking amount by the third week of February, 2019. As per cost sheet issued at the time of booking, I am supposed to clear 5% of the consideration value with 12%GST levy within next fifteen days. Since GST is going to be levied at 5% for my under constructiin flat w.e.f 1.4.2019, I requested the builder to accept my request to postpone my above payment to 1.4.2019 to take advantage of reduced GST by 7%. Moreover if I make this instalment payment now, builder will ask for two more instalments (agreement & foundation) before March 31st 2019 which will work out to another 15% of consideration value. If all the above payments (20%) are postponed to 1.4.2019, I will be saving around Rs.1.2 Lakhs because of reduced GST payment. However the builder is insisting on immediate payment as indicated in his cost schedule. I am in a dilemma whether to pay or not to pay now. Can anyone throw some light on this issue. What builder can do if I do not make any payment now?