I cannot find the Man to whom the Debt is owed
Question :
My brother died and he owed many debts. They have been repaid, and all praise is due to Allâh. Only 400 riyals are left to be paid, so I searched for the person to whom it is owed, but I could not find him. So, what should I do with it? Should it be distributed among the poor or should I deposit it into the public treasury of the Muslims?
Answer:
If the matter is what has been mentioned, in that you searched for the person to whom the debt is owed and you could not find him, then you should give it away in charity intending that the reward will be for that person. If he comes after that (for his money), inform him of that. If he is satisfied with that, that is sufficient. If he is not satisfied, then pay the money to him and you will get the reward (of the charity), if Allah wills. And success is from Allâh. May Allâh send blessings and peace upon His servant and Messenger, Muhammad, and upon his family and his Companions.
Source:
The Permanent Committee
Fatawa Islamiyah, Vol. 3 Pages 432-433