Sale of Agreement

Sir my father had made a sale of Agreement as purchaser in April 2010. And unfortunately he died after 15 days. After 2 month I also paid 50000/-00 Rs to seller. Time was decided for 6 month on agreement deed. But there was occurred some litigation & disturbances to that plot. I was dealing to all this. In October 2012, Seller sold that plot to anyone without any prior legal information to me. I was fully unknown to this. When I knew then I contact & send legal notice to seller & new buyer both but they have not given any response from 2012 to yet 2016. Plot is vacant . They also promise me orally to give half of diffrence money due to more rate paid by new than me but they always begging for some time. Now the seller & new buyer both threating me to lodge an FIR against me if I create any litigation & saying only to pay the agreemented advance amount. Suggest me the best legal procedure bcz m want that plot or decided half of difference money?? the matter & plot is related to patna,bihar.