Is it permissible to postpone the fasting of a vow—which is ten days—from the summer to the winter so that the days may be shorter?

Your friend is obligated to fulfill his vow, for the Prophet, peace and blessings be upon him, said: "Whoever vows to obey Allah, let him obey Him." It is best for him to promptly fast the days he vowed to clear his conscience. This type of vow—one that is conditional—is disliked, for the Prophet, peace and blessings be upon him, said: "Indeed, a vow does not bring a son of Adam anything except what has been decreed for him, but rather it is the decree that prevails over what has been decreed for him, and thereby something is extracted from the miserly." This vow demonstrates ill manners towards Allah. Scholars have differentiated between a vow conditional on a benefit, which is disliked, and a vow that is not conditional but intended to draw closer to Allah, which is not disliked.