How does a person atone for thefts that he repented of, but whose owners he does not know, and will they be considered a debt against him on the Day of Resurrection, to be taken from his good deeds?

One who is compelled to take another's money incurs no sin, but is obligated to return it when able. As for one who transgresses against another's property without necessity, through theft or usurpation, they are sinful, and along with repentance, are obligated to return what they transgressed against, for the Prophet, peace and blessings be upon him, said: "The hand is responsible for what it takes until it returns it." If one does not return it in this world despite being able to, they will repay it from their good deeds on the Day of Judgment.