Non payment of the agreed sum by buyer

I entered into an unregistered Ageeement to Sell with a buyer for selling a floor. The payment schedule mentioned in the agreement is construction linked. Construction is complete as per the agreement but the buyer has not made the 2nd last payment of 15lacs and last payment of 30 lacs to me. I have completed all necessary formalities to get the floor transfered and registered in his name and have served numerous payment reminders to him. There is a clause in the agreement that if the buyer do not make the payment to the seller within the stipulated period then the buyer will pay Rs1000 per day as penalty to the seller for maximum 75 days and on completetion of 75 days the agreement will be automatically cancelled and the earnest money will be forfeited. I have come to know that the buyer intends to sell the floor to someone and hence entered into an agreement with him w.r.t the said floor which is not yet transferred and registered in buyer’s name. The buyer is also forcing me to enter into a triparty agreement with him and the second buyer. What should I do?