Gst on payment made after completion certificate(oc) in rrep

IF Mr.A has booked an under construction flat at total 50,00,000, and paid as advance Rs.20,00,000 and paid GST advance also, but question is after completion certificate arrives and at the time of making purchase agreement GST on balance amount of 30,00,000 is also have to pay or not?? 

Here balance amount of Rs.30,00,000 is to be pay after completion certificate and such Transaction is not liable for GST accoring to Act.