Builder Not Passing GST Benefit To Customer

I have bought an apartment from Sobha builder on 26th Feb 2019 at an agreed price of 5750 / sq feet + 8% GST + 5,19,000 (Additional charges for khata, electricity etc...). I paid Rs, 2L on the same day. I got a confirmation letter (named as information letter with above details on 1st March 2019). However, they did not provide an agreement copy. Later on 10th June, 2019 they sent another information letter with revised price of 6150 / sq feet + 1% GST + 1,29,000 (Other charges). They have unilaterally changed the selling price and did not pass on GST reduction to me. Information letter clearly conveys that any changes in tax will be the customer's responsibility. When I asked them why are they not giving me full GST credit, they told me that I can take money back if I want. I want to explore the legal option. Let me know your point of view.