How to get back the property of my grandfather

Hello, My grandfather had purchased a property in mumbai. He had done Power of attroney and gave rights to my father. My father had to do a sale of agreement to someone when he was having bad financial conditions. My query is My father and that person had not gone to registration office that time. Now the building is going for redevelopment and that person is calling for NOC signature and is giving some amount as the papers still reflect my grandfather's name. I need information on what we can do best in this case to get the property back.