Builder demanding money before time

I am presently working in Nigeria . Last year in June/july 2015 we booked flat at Gurgaon with reputed builder. the terms of payment were 20% in 5 month (December) & issue allotment/agreement copy, 20% on superstructure, 40% on finishing & 20% on possession. The total period of completion was 4 yr after allotment date ( december 2015)+ 6 month. we completed the 20% payment on time in december 2015 before we were issued allotment letter . Just two month later builder informed we have worked really fast & superstructure will be completed by first week of March & we have to pay another 20%. The builder did sent picture as proof. This was really shocking as time taken to complete the superstructure is around 2.5 to 3 yr , which was also advertised online & discussed before we agreed to buy. we wrote back saying we cannot afford to pay 20% in three month time as terms initially discussed were to pay after 2.5 yr when superstructure is complete. Even agreement was pending which we got just today after lot of request made. The allotment letter was received in December. I even requested them to give us time for completion for each milestone so that we can make necessary fund arrangement now & future. They refused saying they don't have timeline now & will share later. The property agent confirmed all other customer have complained against this behavior by builder as have have asked everyone to pay. The builder is saying he will forfeit the entire 20%. The demand letter is yet to come. Really need an expert advice from highly professional real estate/property lawyer.