Procedure in case No possession given after registration by builder

Builder not willing to give the keys unless i pay future 30k maintenance in advance. The amenities are not yet complete - only power backup and security is there. I am planning to start interior work next year and shift much later. if i don collect the keys until next year, can the builder still force me to pay the due maintenance? How can i protect myself legally? Builder has not kept me informed, he said “sir pls come and collect your keys”. When I went he said only after you pay maintenance you will get keys. This was shameful as I suddenly cant arrange for 30k. It is his job to keep me informed in advance so that I can make arrangements. I find this like harassment. Even for registration he didn’t inform me of the 20K bribe amount required, after signing the documents is asking for 20k otherwise registration will not happen. I had to suddenly borrow from a friend and pay the bribe. Even on the reg. day when I asked him about the keys he did not mention about the maintenance cost.