Purchased land paid advance owner sold land to someone else

Sir, i purchased a plot from a mediattor for 8 lakhs annd made an agreement on 100 rs stamp paper from the plot owner, saying that he will register the plot after he gets registered from society,as it is a society land. but without my notice the mediator and owner as sold plot to other party, how to get my property ,is the agreement valid or not its almost 8 years.