This is another article in the chapter Justice from the classical manual on Sharia called Reliance of the Traveller. The articles are posted one by one in the order they are given in the book.
021.0 THE JUDGESHIP
021.1 To undertake the Islamic judgeship is a communal obligation (def: c3.2) (0: for those capable of performing it in a particular area). If only one competent person exists who can perform it, then it is personally obligatory for him to do so. If he refuses, he is compelled to accept (0: though he is only obliged to accept the judgeship when it is in his own home area, not when it is elsewhere, for this would be like a punishment, involving as it does wholly leaving one’s home). Such an individual person may not take a salary for it–(N: because in respect to him it has become personally obligatory, and it is not permissible to take a wage for something personally obligatory, as opposed to something that is a communal obligation (A: for which accepting a wage is permissible ))-unless he is needy (0: in which case the Muslim common fund gives him enough to cover his expenses and those of his dependents, without wastefulness or penury. But if he agrees to judge without being paid (N: i.e. in expectation of the reward from Allah), it is better for him).
021.2 It is permissible to have two or more judges in the same town.
021.3 It is not valid for anyone besides the caliph (def: 025) or his representative to appoint someone as judge.
021.4 It is permissible for two parties to select a . third party to judge between them if he is competent for the judgeship (def: 022.1) (0: provided the case does not concern Allah’s prescribed penalties, (A: and they may select such a person) even when a judge exists). It is obligatory for them to accept his decision on their case, though if either litigant withdraws his nomination before the third party gives his judgement, the latter may not judge.