How should one deal with money taken unjustly, when it is impossible to return it to its owners, and its possessor wishes to repent from it, and fears accountability if its source is disclosed?

You must repent of the sin by regretting it, abandoning it, and resolving not to return to it, as Allah Almighty says: "And turn to Allah all of you, O believers, that you may succeed," and as the Prophet, peace and blessings be upon him, said: "Regret is repentance." Since the sin pertains to the rights of people, repentance is not complete إلا by restoring the rights to their owners or their heirs. If they cannot be found after searching, then give it as charity on their behalf. If they appear afterward, they have the choice between being content with the charity or reclaiming their right. It is recommended that you conceal yourself and not inform anyone.