Praise be to Allah.
You have to ask the people who know him and make the effort to find out where he is, then send the money to him, or else go and see him and give him what is due to him, because the Prophet (peace and blessings of Allaah be upon him) said: “Every Muslim is sacred to another Muslim in his blood, his wealth and his honour.” It is not permissible for you to take the matter lightly. If you are unable to find him, then give the money in charity on his behalf, with the intention of repaying him too. If you then find him, then give him the choice between getting what is owed to him, in which case you will have the reward of giving in charity, or accepting the fact that you gave it in charity, in which case the reward will be his. Allaah says (interpretation of the meaning):
“So keep your duty to Allaah and fear Him as much as you can” [al-Taghaabun 64:16]
“Allaah burdens not a person beyond his scope” [al-Baqarah 2:286]
Shaykh Ibn Baaz (may Allaah have mercy on him).