What are the rules concerning the appointment and concurrence of the arbitrator?

Primary text

If two men are appointed as arbitrators, their ruling is valid if they agree. If the Imam appoints the arbitrator upon the people's request, it is valid. However, if the inhabitants choose the arbitrator themselves or the designation is left to them, it is not permissible unless the Imam approves the chosen individual, as seen when the people of Banu Qurayzah chose Sa'd ibn Mu'adh, whom the Prophet, peace be upon him, approved and whose judgment he upheld, stating, "You have judged concerning them with the judgment of Allah."