What is the ruling on not fulfilling a vow to fast ten days and donate books, and what must the one who made the vow do?

You must fulfill a vow contingent on a condition if the condition is met, based on the saying of the Messenger of Allah (peace be upon him): "Whoever vows to obey Allah, let him obey Him." Fasting is an act of obedience. Therefore, if you vow to fast specific or non-specific days, you are obliged to fulfill it. If the days are not specified, you may postpone them until it is convenient for you, but it is better to hasten the fasting to clear your conscience. This type of vow is disliked, because a vow does not change anything from destiny.