Builder Demanding Interest On Delay In Payment Where In There was No D

I booked a flat wity tye builder in July 2018 for 1.2 cr by paying 46 lac bookkng amount after that builder was demanding more money and i had to go to court to get the property refistered.. the case result was in my favour and builder had to execute the agreement. I made the remaining pay within 10 days of registration, the case was in rera for 6 months, now at the time of possession the builder is demanding interest for the delay of 6 minths of payment stating it is the standered practise to charge interest if the delay is more than 45 days, which is just inapropriate the interest chareged is 15 lac, i dont want to get stuck the legal matters again as i need the possession, is there any faster way to resolve this