How to cancel registered Agreement.

I had purchased land 5000 sq feet thirty years ago from the owner who owned the whole gata.In the "bainama" which was done the owner has given "rastey ki zamin" around 20 feet on the frontage of the plot as per the naksha in the bainama.A similar bainama was done with the other two buyers of plots in the same gata and rastey ki jamin was given to them .The seller had no land left in his possession.But the land record still showed some 1500 sq feet of land in the original seller name.Thirty years after I had originally bought the plot the seller of the plot sold his 1500 sq feet of "rastey ki zamin " to a property dealer as he had this land on paper in the record and the two entered a registered agreement for the same. This property dealer tried to do construction on my site forcefully. What action can i take to get the registered agreement cancelled. How can i proceed to get the land record corrected How to prevent the 1500 sq feet land to be mutated in the property dealer name.