Is a single act of charity sufficient for past sins or as an expiation for an oath (kaffarat yamin) for someone who repented of a sin then repeatedly returned to it, and pledged to give charity every time he returned, until it became difficult for him?

A vow is contracted by every utterance that signifies commitment as an act of drawing closer to Allah, such as vows of righteousness like prayer, fasting, and charity, and vows of obstinacy and anger, like saying, "If I ever return to sin, I will give such-and-such in charity." If the vow falls under the category of a vow of obstinacy and anger, then the one who made the vow must repent upon breaking it, and then he is given the choice between fulfilling what he vowed or offering an expiation for an oath. His vow is thereby dissolved, unless the wording of the vow indicates repetition, in which case he must fulfill it or offer expiation every time the sin occurs. The expiation for an oath is feeding ten poor people, or clothing them, or freeing a slave; if one is unable to do so, then fasting for three days. The questioner should not despair of Allah's mercy, for Allah forgives sins no matter how often they are repeated, as long as the servant repents.