Permissibility of fighting those who break a treaty; Permissibility of letting besieged people surrender, subject to the judgement of a just person who is qualified to pass judgement

It has been narrated on the authority of 'A'isha that Sa'd's wound became dry and was going to heal when he prayed: O God, surely Thou knowest that nothing is dearer to me than that I should fight for Thy cause against the people who disbeliever Your Messenger (ﷺ) and turned him out (from his native place). If anything yet remains to be decided from the war against the Quraish, spare my life so that I may fight against them in Thy cause. O Lord, I think Thou hast ended the war between us and them. If Thou hast done so, open my wound (so that it may discharge) and cause my death thereby. So the wound begin to bleed from the front part of his neck. The people were not scared except when the blood flowed towards them, and in the mosque along with Sa'd's tent was the tent of Banu Ghifar. They said: O people of the tent, what is it that is coming to us from you? Lo! it was Sa'd's wound that was bleeding and he died thereof.