Denying of possession

I have booked a flat in Jan 2013 and as per agreement he has to handover to me within 3 yrs including grace period but during sale deed registration of under construction flat he demanded penalty from all buyers by stating you have not paid the money as per their demand letter date whereas their work progress is actually was not as per their demand and we all buyer were blackmailed and to get our flat registered many of us has paid their demand against our wish and I have also paid post dated cheque of Rs 52250 in the name of development charge which was not mentioned in our agreement and did our property registered but later after PDC clearance at bank it was returned back by bank as mistakenly amount in words were verying with the amount mentioned in numerical value and still they did not recieve occupancy and completion certificate from authority and now for income tax return purpose I need possession certificate but they are denying it in the name of outstanding amount also after getting these unlawfull demand I am not willing to pay further out of agreement demand and that too if I will pay then they will issue possession letter without getting occupancy certificate and the same time they are sending monthly maintenance demand notice and in these years I have already paid more than 10 lakh as interest for against building loan.Please advise and guide me for what to do in this situation.