Is the builder allowed to take 96% payment of the flat value 1.5 years before possession date?

Can the builder take 96% payment of total flat 1.5 years before possession date from the customer and leave only 4% payment of the total value to be paid at the time of possession? Is this legally allowed? I bought a flat in Thane in April 2016. Builder has already made me pay 84% of the total consideration value. Since June 2017, I have been getting demand letters for payment of more than Rs. 5 lakhs every month. Builder says they will ask for the complete consideration value within the next 3-4 months. This would leave only 4% of total value to be paid at the time of possession. As per the agreement, possession is in September 2019 which is more than 1.5 years away. My bank loan has already got over and I'm supposed to arrange the remaining from my personal finances in the next 3-4 months. Builder insists on getting the total consideration value by early next year but say that can't give possession before Sept 2019 because of internal work. I'm supposed to pay 96% of the total value of the flat 1.5 years before possession and keep paying the bank loan EMI on that as well without being able to claim even tax benefit on that since I do not have possession of the flat. Is the builder allowed to do this? Please advise