Buyback deal on plot purchased

Sir, I had purchased a plot at solapur from one of the Pune based builder. The said plot is currently registered on my name as I have paid the full amount at the time of purchase.. The plot was sold to me with buy back guarantee with 100 % return ion the completion of 4 years.. A saperate buy back agreement was made on 100 RS. Stamp paper but not registered. PDC also given to me by builder to provide more confidence.. The four years will be completing in April 2019.. I have started reaching out to builder to understand the exact procedure, require paperwork etc but builder seems to have declining on his buyback commitment now.. The buyback was offered in a way that builder will take responsibility to search buyer who will be ready to purchase a said plot on double the rate of the purchased price. Builder now saying that market is very down and will not get any buyer to buy on the said rate and saying that we can't stick to the commited date as per agreement and asking me wait and watch .... I m not sure what options do I have what now? I think the only positive point I have is I have PDC .. Please guide. Atul Samani