About OC of a property...

I had purchsed a commercial property at a prime location in Thane (W), Maharashtra in the year 2000. The developer had completed two stories & was confident he will get permission for 4 more. But, that never happened, he could complete 4 instead. He also occupied two stories for his own offices little later. I being a professional was uncomfortable with'Illegal constructio' tag on property tax bills every year. I insisted on OC & society formation with the developer. He only made verbal promises but nothing happened. Now, the person is no more , his son who had taken over has committed suicide just a year ago & grandson, an absolute novice, is holding the reigns of the business. I wanted to consider selling the property & when insisted on at least an NOC from him, he has started making demands for money. I have clearly indiacated to him that the firm is at fault & I would move a court if I don't have my rights respected. His infamous father has allowed a telecom to erect a tower on the fifth floor & also has consrtucted some make shift office cabins there. What is the legal remedy ?