Regarding ownership of redeveloped property

Hi I own a flat of 180 sq ft (Carpet area)in chembur camp(First party- Sannat Holder). I sold a part of it (40 sq feet) in 2004 via a sale agreement. The area is fraudulantly mentioned in the agreement as 80 sq ft which i came to realise now, as at that time I had encountered many distressful situations(My Parents had expired and My brother Expired too, also i was subjected physical and mental Harrassment from my Husband which led to our divorce). Now the building is going for redevelopment. The purchaser of 40 sq ft is not giving me a buyback option 1) Am I liable to give the purchaser of 40 sq ft a 22.22% share in the larger redeveloped flat which I'll receive? 2) what if as per the fraudulant mention of 80 sq ft in the sale deed if he demands 44.44% share, then what are my options? 3) at the time of society formation if only my name goes in the member list then is he liable for a claim? 4) The Developer is asking me to sign an irrevocable development agreement, in emergency situations can this agreement be broken.