Praise be to Allah.
One of the principles of good manners which is supported by human nature and which Islam confirms is of great importance is rendering back trusts and abhorrence of treachery and betrayal. Allah says (interpretation of the meaning):
“Verily, Allaah commands that you should render back the trusts to those, to whom they are due; and that when you judge between men, you judge with justice. Verily, how excellent is the teaching which He (Allaah) gives you! Truly, Allaah is Ever All‑Hearer, All‑Seer”
“O you who believe! Betray not Allaah and His Messenger, nor betray knowingly your Amaanaat (things entrusted to you, and all the duties which Allaah has ordained for you)”
On this site we have previously explained that it is haraam to take the wealth of kaafirs by deceitful means. See the answer to question number 50716. We have also explained the obligation of returning stolen wealth to its kaafir owners. See the answer to question 47086 and also 7545, 14367 and 31234.
Whoever falls into wrongdoing, robbery or consuming people's wealth unlawfully then repents from that is obliged to return to each person that which is rightfully his in this world
It was narrated that Abu Hurayrah (may Allah be pleased with him) said: The Messenger of Allah (blessings and peace of Allah be upon him) said: “Whoever has wronged his brother with regard to his honour or anything else, let him seek his forgiveness today, before there will be no dinar and no dirham [i.e., the Day of Resurrection], when if he has any good deeds (hasanaat), some of his good deeds will be taken and given to the one who was wronged, commensurate with the wrong that he did, and if he does not have any good deeds, some of the bad deeds (sayi’aat) of the one who was wronged will be taken and thrown onto him.” Narrated by al-Bukhaari (2269).
And it was also narrated that Abu Hurayrah (may Allah be pleased with him) said: The Messenger of Allah (blessings and peace of Allah be upon him) said: “All rights will be restored on the Day of Resurrection, until even the hornless sheep will settle its score with the one that has horns.” Narrated by Muslim (4679).
But if the one who is in possession of the property does not know who the rightful owner is or where he is, or he is unable to get it to him, then he should give it in charity on his behalf, then if he gets in touch with him the owner of the property or wealth is to be given the choice between having the reward of the charity going to him or having his wealth returned to him and the reward for it going to the one who gave it in charity.
Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him) said:
If he knows that his wealth includes permissible wealth and haraam wealth but he does not know to whom it belongs, but he knows how much it is, then he should divide the wealth according what is halaal and haraam and take the halaal portion; the haraam portion should be given in charity on behalf of its real owners, as should be done by one who possesses wealth whose owners are unknown, such as money acquired by force, or things that have been borrowed or entrust to him. The majority of scholars, such as Maalik, Abu Haneefah, Ahmad ibn Hanbal and others said that he should give it in charity. This is what is narrated in such cases from the Companions of the Messenger of Allah (lessons and peace of Allah be upon him). If it is not known how much is halaal and how much is haraam, then he should divide the wealth in half, keeping half for himself and giving the other half to its owners if he knows them, otherwise he should give it in charity. What he gives in charity should be spent on the general interests of the Muslims: it may be given to those who are entitled to zakaah, or spent on guests, or used to help pilgrims go for Hajj, or be spent on jihad, or other charitable causes that Allah and His Messenger love, as is done with all wealth whose owners are unknown. This is what should be done by one who repents from doing haraam things and has haraam wealth in his possession and does not know who its owner is. End quote. Majmoo al-Fataawa (30/328)
See also: Majmoo’ al-Fataawa 28/592 ff.
Shaykh Mustafa al-Ruhaybaani (may Allah have mercy on him) said:
He has to give it in charity (when there is no trustworthy ruler), as is the case nowadays. But if there is a trustworthy ruler, which is more rare than red sulphur, then in that case he does not have to give it in charity; rather he has the choice between giving it to him [the ruler], so that he will no longer be responsible for it, or giving it in charity. That is a sound view, provided that he takes responsibility for it before the owners if he finds them, because giving it in charity without assuming responsibility for it is a waste of the owner’s property. The is the ruling concerning stolen property and the like, such as lost property that he should not have picked up and he did no announce; in that case he may give it in charity on behalf of its owner, provided that he will still take responsibility for it before its owner.
Mataalib Ooli al-Nuha, 4/66
And Allah knows best.