Builder demanding payment which is not as per line of Contract

Hello All I have bought a flat in a under construction project with a payment plan that to pay 15% on booking, 25 K each month till project completion ( June 2025) and 3 lakh every 6 months and rest amount on the day of the possession which is projected as last payment in June 2025. The project is in stages and seems the buiilder is completing this section earlier than expected and demanding a 90% payment now and sent a demand letter. also No ameneties done and some work pending but I may be they will complete in coming 3 to 4 months. But I am not prepared for it as I have bought only on that condition of payment which they advertise hevely and mentioned in their prospectus and also in my contract. Now I am in deep stress as how to pay a money which a year in advance as I have planned my finances on it. I can't take loan and don't want to. what are solutions I have? Do I have to pay..even though the contract said otherwise? My contract is well designed and mentioned each month of payment and last payment on June 2025. Thank you Regards