Land ownership. Illigal possession of land.

My grandfather have a plot, which got transffered to my grandmother after granfather's death. My distance relative(grandfathers brothers son) has occupied it. Initially he took keys from us for temporary use and now he is refusing to give it back. We approached police, when police asked him, he presented a paper. It states my grandfather took 1L from his father who is also my grandfathers brother and if we return him back 1L then only he will give us our possession back. My grandfather died in 2006 Then lender ( grandfathers brother ) died in 2000 And he never asked for the money ever nor we were even aware of this fact .. but when we approached court he presented that paper. What should We do now? My grandfather was illiterate and they used him ... they were planning for my grandmother to die so that they can claim the possession of land based on that paper. Please advice.