Builder demands extra money before possession even though I paid full at one time.

I purchased a flat in May 2022 worth ₹35 lacs in ₹30 lacs(+extra 1 lakh 50 thousand for stamp duty), because I negotiated with the builder ( which he agreed happily to), as I paid the full 100% amount in one transaction and the flat was under construction. The builder had promised the flat within 6 months but failed to do so. Cut forward to 1 year and 7 months later i.e. December 2023, the builder demanded ₹89 thousand(₹35,000 for electricity + ₹15,000 for water+ ₹15,000 GST+ ₹24,000 for advance maintenance charges) before possession. But I occupied the flat from December 25th without paying that money. But now he daily demands for the money. I have the sales deed but no occupancy certificate, no power supply meter in my name. But here's the problem, I am the only one that this demand( those extra charges) has been asked to meet. When I asked the reason he said that because he sold them (other consumers) the flat for ₹35 lacs and that included all the charges and thus provided them all the services for that money. But again, here's the thing other flat owners purchased the flat just a few months back in an almost complete situation, and they are going to pay the amount in stages, and not the full amount in one go. And they already got their power supply meters. I asked him that this is completely wrong because he sold me the flat 1 and a half year back, in that amount only because it was going to be a one time payment , and too for an under construction flat, and that he could have mentioned this at the time of the deal. I feel this is ridiculous and he is fooling me. What should I do?