Part payment for property. Now owner died and heirs not wiling to sell

d contract made for consideration of 3,50,000/-,my dad paid 1,50,000/- to d defendants father, before the completion of contract d defendants father were died, my dad after d death of defendants father gone to pay remaining money, but they made reasons for avoidance, my father put a case against them and the judgment was their is prima facie case and no legal injury was cause to the plaintiff then what is the remedy for my dad? and that place is for the public purpose my dad wants hat place for (kamliwale baba seva samiti) making madarsa....will you plz help me for the succession in this case