Kartik estate and Paramount builders – False promise and unfair trade

I bought a flat from Paramount builders through their channel partners ( Karthik Estate) in paramount floraville, sector 137, Noida greater noida expressway. I paid around 500 Rs per square ft extra ( under subvention scheme of 37% now and 63% on possession) on the condition that the builder will not demand remaining money before 18 months ( possession). Now I have got the contract and there is a very tricky condition in it saying that builder can demand 63% money within 18 months ( means anytime from paying 37%). i bought the flat on this subvention merit paying 500rs per square ft extra but I am cheated as I have paid extra but still not protected and builder can demand anytime. I have approached both builder and Karthik estate many times but to no avail as both of them have flatly refused to honor it. It unfair practice as well as the ad of the builder is misleading. the customer can enter in this scheme because of this attraction but the builder still reserve the right to demand anytime.. Request you to kindly intervene as i need fair deal.