When I am angry, if I swore not to do something that is halal, is it haram to do it after this oath?
All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
If a person swears not to do something lawful, it is permissible for him to fulfill his oath and give up doing that lawful thing. It is also permissible for him to do that lawful thing and expiate for breaking his oath. However, it is better for him to fulfill his oath as Allah Says (what means): {And fulfill the covenant of Allah when you have taken it, [O believers], and do not break oaths after their confirmation while you have made Allah, over you, a security [i.e. witness]} [Quran 16:91]
However, if one sees some benefit or advantage in breaking his oath, then he should do whatever is good and favorable to his interest and expiate for breaking his oath as the Prophet said: “If you make an oath and (later on) you discover that doing something else is better, then you should do that and expiate for breaking your oath”. [Al-Bukhari and Muslim]
As regards anger, it has no effect on one’s oath. The angry person is responsible of his words and actions unless he is angry to an extent that he is unconscious and does not realize what he says, and faints; in this case, he is considered as an insane person because he has lost his mind. So, he is not held accountable of his oath.
Allah knows best.
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