Creating third party rights bypassing a registered housingsociety

Dear Sir, Ours is a registered housing society located in Mumbai city. Our society consists of one building having three wings. It was build 30 years back with F.S.I of 1.33. Two wings were fully completed and the third wing was not constructed. At that time there was also a tenant chawl (cess) which was not supposed to be developed, hence it was retained as F.S.I was totally consumed. As per his obligation he had to complete the third wing and hand over the conveyance of the plot to the society. As per the current status he has no intention to complete the structure. Due to which our building is termed as incomplete. We have complete OC of the entire two wings. During registration of the society various undertakings were given. In which he had mention that he will complete the construction and will execute the conveyance deed along with all structures existing on the plot. But till today he has not done anything. Now he wants to redevelop the chawl which liable for additional F.S.I being a cess structure. Recently we came to know that our builder has sold the entire plot to a developer. He has also executed a conveyance deed with the developer. Can the owner execute a conveyance deed to a third party without giving us the conveyance as per MOFA Act? Kindly enlighten us and what steps should we take to protect our right.