Builder asking me to pay tax before gst implementation

First of all please check your allotment letter/builder buyer agreement whats is the payment plan in it and before GST how much amount is due in payment plan if its 20% then the amount of 20% is charged in 4.5% and in possesion if its 80% then the amount of 80% comes under 12% rate of GST.

in my opinion if you pay the advance to the builder then the tax will be charge 4.5% before GST but afterwards it will charge 12% on balance due amount.