What is the ruling on accepting financial aid from one's sister after the husband told his wife, "You are forbidden to me if you take any financial assistance from your sister," in a moment of anger, and then claimed he only intended it as a threat?

If a man forbids his wife, the ruling depends on his intention. If he intended divorce, it is a divorce. If he intended ẓihār (a pre-Islamic form of divorce by likening one's wife to one's mother), it is ẓihār. If he intended an oath, or did not intend anything, then he must offer expiation for breaking an oath (kaffārat yamīn).

Since the condition on which he swore the oath has been met—namely, taking help from the sister—and the husband explicitly stated that he only intended to threaten, he must offer expiation for breaking an oath. The husband is advised to expiate his oath and take the help, based on the Prophet’s (peace be upon him) saying: "If one of you swears an oath and then sees something better than it, let him expiate it and do that which is better."