Is it permissible to reject the decision of the judge who ruled between the two partners and resort to another, especially after it became clear that it violates Islamic law, particularly after a number of muftis confirmed that the sale was concluded and cannot be reversed except by mutual cancellation?

If the partner offered to buy your share, and you accepted the deal and blessed it, then the sale is valid and binding. He cannot retract it except if you release him from it, because a sale is based on consent, and it was completed at the contracting session.

As for the ruling of the one you referred your dispute to, the extent of its validity cannot be determined except by knowing his capacity (whether a judge, an arbitrator, or a mufti) and the extent of his competence, and whether he based his ruling on a recognized jurisprudential opinion.

Be aware that the ruling of a judge and the fatwa of a mufti are based on outward appearances, and they do not excuse someone who knows that the inner reality of the matter is otherwise.