I Lible to pay GST ???, If yes How % it ???

Hi, GST Experts,
Please guide me.

I am a Land Owner. I give a plot (1503 S.F.) to Developer for Development with a Development Agreement. Here is Doable FSI so
Developer Aggrid to give me 2050 S.F. Sellable Ready Construction.

Important - Here is no any Amount Give OR Take.(Without Refundable Deposit)

1) First Development Agreement & Power of Attorney Date is 07/05/2016

2) Mobadala RS 42,40,000/-
(It's only shown for Gov.stamp duty)

3) Development Agreement of Developer & Me , Developer writing in the agreement that vat,tax,GST pay by writing giver.

3) Our GST Law Start from July 2017

4) Completion certificate issued
from PCMC Carporation Date 20/05/2019

5) Till Today Our Final Agreement is not done .

6) Developer demand me to pay GST 18 % = (7,63,200/-) , on our Mobadala RS 42,40,000/- (It's only shown for Gov.stamp duty)

7) So I Confirm from you That I Lible to pay GST ???
If yes How % it ???

8) My Mobile number is 9850903917 if any fees I will be pay .

9) With Thanks & reply me as soon as possible.

10) Regards with Nikam Kisandeo.