What should I do, as I vowed to give 100 dinars to someone, but I could not find him and have forgotten his face?

Undertaking a vow is disliked (makruh), and whoever vows obedience to Allah must fulfill it. If you are unable to fulfill your vow because the man to whom you vowed to pay the amount does not exist, and you do not know who he is, then it is sufficient for you to pay an expiation for an oath (kaffarat yamin). Some scholars have stated the necessity of an expiation for an oath in cases similar to this issue, as Ahmad said concerning someone who vowed to emancipate a specific slave, and the slave died before he could emancipate him, an expiation for an oath is incumbent upon him.