Is it permissible to pay a bribe to a judge to reclaim agricultural land that was possessed by an 'urfi (customary) contract, which is legally void, after money and effort have been expended on it?

The lack of notarization of contracts does not affect their validity. What the seller wants—to seize your land—is an injustice. There is no blame upon you for defending yourself against it by whatever means you are able. If you cannot get rid of him except by that means, then the sin falls upon the one who took it, not upon you. This is because bribery is what is given to nullify a right or to uphold a falsehood. As for what is given to obtain a right or to repel an injustice, it is not considered bribery.