Delay possesion by builder and refuse to pay rent

Dear Sir, I purchased flat in Dombivali (Thane, MH) in 2017. WHile booking RERA was not in place and builder promised Dec 2018 as possesion date. Time of registration RERA was in place and builder mentioned Possesion date in RERA as Dec 2020. I put condition before registartion that I should get compensation for delay possesion, for which Builder created supplimentary agreement mentioning 10k Per month rent from Jan 2019. Same thing he confimred in mail in Feb 2020, but said he will pay compensation at time of possesion by adjusting payment I've to do at time of possesion. When I asked him for rent payment in Oct 2020, as all of my balance payment will be less than rent amount, and asked him to continue rent payment. In Jan 2021 he replied he wont pay anything and he's willing to cancel booking. Could you please guide next steps I can take against builder for this?