Is a person considered a usurper of land that his neighbor claimed ownership of, even if the judiciary ruled the neighbor's claim to be null and void?

The burden of proof is on the claimant, and the oath is on the one who denies. If your neighbor proves his ownership of a part of your land with evidence, then he is entitled to what he has proven. You would then refer back to the seller for the value of the shortage, or return his land if you wish. If he does not have evidence, then the land is yours, and you are not an usurper, unless you know his claim to be true. If you have built on the disputed part and the claimant proves his claim, then he has the right to give you the value of the standing structure. If he refuses, then you can pay the value of the land. If both refuse, then you will be partners in the land and the building. In all cases, you refer back to the seller.