What should one do who vowed to donate a sum of money to a poor person if they committed a sin, then committed it but could not find the poor person to give them the money?

Abandoning sin is obligatory. If a Muslim vows to abandon a sin, then abandoning it becomes even more emphasized for him, and he is not permitted to break his vow. If he breaks his vow and commits the sin, he must repent. He has a choice between fulfilling his vow or offering an expiation for an oath (because it is a "vow of obstinacy"). If he chooses expiation and intended to give charity to a specific person but cannot find them, he may give the amount to another poor person whose need is similar to or greater than the need of the one he initially designated.