Sale deed

Respected Sir, I want purchase a flat. Owners of the flat are four brothers where the apartment is situated. The builder (Goutam Das) is a civil engineer. He is a central government employee. His business is in the name of his wife (Suparna Das) the apartment is G+3. Three two bhk flat in each floor and at ground floor one two bhk and two car parking area. To get maximum floor area ratio or extra one flat at ground floor the had put his name as a tenant in the sanctioned plan at ground floor plan. But he was never a tenant or neither owner of that ground floor flat. After completion of total building he gave in written declaration in white page that he would not wish to stay there. He also declared that he did not any claim for that flat. Now if I purchase that flat without tenant's signature i.e confirming party (Goutam Das) what problem may be arise. Can he any claim for that flat? Is there no value of that written deceleration in white page? Please answer me as soon as possible. It's very urgent.