Sale deed without prior agreement to sale

I am buying one flat from investor. Investor had bought 2 flats from builder and he is selling second flat to some other person. Myself and investor agreed on normal payment terms. Later builder said to investor that according to ATIB tax benefit law, builder can not sell more than 1 flat to single person. Investor told me that he will cancel 1 flat and return it to builder and later I will directly buy it from builder. Our payment terms remain same. I have few questions. 1. What is ATIB tax benefit law? 2. Investor says that I can directly go for sale deed without first going for agreement to sale. Is this correct?