A dispute between the owner and the dealer over the property.

Few days back. My grandpa wanted to sell a piece of his land to the dealer to make the building / flat. The dealer finally came with the agreement paperwork, the family members didn't like to get agreed on few things like : parking, the 6th floor to be build (we don't know if it's even legal to proceed on 6th floor). Dealer wanted the whole parking and what not. We marked them, and tried to resolve with him, and asked him to make the new agreement without pen/pencil corrections. He said it'll work. And in the meantime he re corrected and dismissed our correction's and unknowingly of that. My grandpa signed on the agreement, now the question is either he wants the dealer to cancel the deal or resolve the issues, mentioned. But the dealer is neither agreeing on canceling the deal nor, resolving issues. He says the deal is now done. And you cannot do anything about it now... I would request you to let us know if this is really true? Can't we really cancel the deal? Kindly help.