What is the ruling regarding someone who made a vow to perform something if their wish is granted, but their wish has not yet been granted, and what is the expiation for not fulfilling it?

A vow contingent on the occurrence of something becomes obligatory only when that thing materializes. Fulfilling it is unanimously obligatory, based on the Prophet's (peace be upon him) saying: "Whoever vows to obey Allah, let him obey Him." If fulfilling the vow becomes obligatory, it becomes a debt upon the one who made the vow. Abandoning it is a sin and an offense, and Allah Almighty may punish the one who neglects it with hypocrisy in his heart, as stated in Surat At-Tawbah. Nevertheless, making vows is disliked, due to the Prophet's (peace be upon him) prohibition of them and his explanation that a vow does not avert anything, but merely extracts from the miser.