Owner demands more money after sale agreement of flat

I purchased a flat and made agreement in feb 2019 and paid 6lakhs,which is valid for 45 days with in which have to register the flat and agreement states that we are allowed us to take possession and we took possession in march 2019. later when we want to register we found out that occupancy certificate is not issued yet so we need to bear the GST 6% as under construction project although its completely finished and many other flats are occupied and registered. so we decided to wait for occupancy certificate as builder promised to get it in 45 days, and it went on till December 2019, but we didnt receive the OC, so we were fed up and decided to register in December 2019 and renew the agreement to 90 days for loan process, while the bank came to validate the home in loan process they found out that our flat was in mortgage and we were shocked because as per builder and owner other flats were in mortgage not ours. so we couldn't register our flat and keep stressing the owner and builder for oc to proceed further. finally when we received OC on july 2nd 2020, when we reach the owner for renew sale agreement for registrations and loan process, he is demanding extra money as rental for occupying the home till date and saying we are staying for free these days in his flat, which was shocking as he was keep forcing them for oc later which we found its in mortgage now to proceed further do i need to pay extra money? firstly he cant sell me mortgage flat which is illegal before., now that mortgage is cleared he is showing his true colours. please help what action can i take,