May a sick person pray the regular supererogatory prayers while sitting? And is it permissible for someone who started a voluntary prayer standing to sit and complete it while sitting if they feel tired during it?

It is permissible for a sick person to pray the obligatory prayer sitting if they fear an increase in their illness, or a delay in their recovery, or if standing causes them severe hardship, or if standing diminishes their devotion (khushu'). If they begin the prayer standing and then become unable to stand, they should sit and complete their prayer. As for supererogatory (nafila) prayers, it is permissible to pray them sitting even if one is able to stand, with the reward reduced by half if there is no excuse, and the full reward if there is an excuse.