Do i need to pay GST on purchase of flat whose amount is paid before the start of project

Hi Sir/Madam, I had purchased a flat worth 3.25 cr in the year 2018,and had paid 85% amount to builder before completion. However due to some internal dispute between builder and land owner,the property was never handed over to me. So in 2022 he offer me a deal where he made a sale agreement that he has purchased the earler flat from me at a price of 4 cr and sell me a new flat at 4.0 cr. that he will build a new apartment and will adjust this amount there and shall deliver the flat in his new building. He asked me the balance 15% àmount at the time new agreement before the start of new project.GST payment was never discuased at anh time during the inital deal till recently.TO SUMMARISE IT he has taken 100% payment from me before the before of project Now after 18months when the property is ready he is asking GST on it. The follwongare my queries. 1. Do i need to pày GST in such case? He is asking me 32 lacs extra for GST. I dont understand how is he calculating such Amount. How much actual GST do i need to pay if required. 2. I also have a fear that he is not going to deposit this GST andhe actually trying to gain more profit in pretext of GST. How i ensure that that he must pay correct GST to govt.?