The Ruling on One Who Made a Vow Dependent Upon Something in the Future and it Did not Happen

Question :

A person vowed that if something forbidden occurred, he would pay a month's salary to a certain person. Since this forbidden thing did not occur, is there any atonement incumbent upon him, or should he pay the month's salary. And may the atonement be paid in money, giving twenty Riyals to each poor person?

Answer:

Whoever made a vow with the condition of something occurring in the future, but it did not occur, then he has not broken his vow, there is no atonement incumbent upon him, nor is he required to fulfill the vow. But when the attached. condition occurs he must fulfill the vow, if it is a vow to do an act of obedience. For example if he said: "If I make a profit, I vow to give one month's salary as charity to the poor for Allah's sake."

If the vow was to an act of disobedience, such as his saying: "If such-and-such a thing happens, then I must drink a glass of wine," or some other such thing, then fulfilling it is forbidden. and he must atone for an unfulfilled oath.

If the vow was merely to do something lawful, such as his saying: "I must buy such and such a garment or such and such a type of car," it is permissible for him to fulfill the vow, or make atonement for an oath. That is, to feed ten poor persons one meal equal to the average meal he would feed to his family, or to clothe them, or to free a slave. If he is unable to do these things, then he must fast for three consecutive days and it is not sufficient for him to give the value of the food.

Source:
Ibn Jibreen.
Fatawa Islamiyah, Vol. 6 Pages 437-438