What is the religious ruling regarding breaking an oath not to take money that was given as assistance for a brother's medical treatment, but was later accepted due to straitened circumstances?

There is no harm in swearing by Allah or His attributes to affirm something, and if a person breaks their oath, expiation becomes obligatory upon them. You have broken your oath by taking the money, and there is no harm in breaking an oath if something else is better, based on the Prophet's (peace and blessings be upon him) saying: "Whoever takes an oath and then sees something better than it, let him do what is better and expiate for his oath." The expiation for an oath is: feeding ten poor people, or clothing them, or freeing a slave. If one cannot find any of these, then fasting for three days.