How can an oppressor repent and restore injustices to their rightful owners if he is unable to do so, such as if he has retired and is no longer able to return rights to their owners?

For repentance from sins related to the rights of people to be accepted, injustices must be returned to their rightful owners, as the Prophet (peace be upon him) said: "Whoever has committed an injustice against his brother, let him seek his forgiveness for it, for there will be no dinar or dirham (currency) there (on the Day of Judgment); before some of his good deeds are taken for his brother. If he has no good deeds, then some of his brother's bad deeds will be taken and cast upon him."

If he took money, he must return it to the wronged person or to their heirs. If it is impossible to reach the wronged person, he should give the money in charity on their behalf. If he is unable to pay the money or seek forgiveness, he should repent to Allah, and Allah may fulfill the right on his behalf on the Day of Judgment.

As for material rights, they must be returned to the wronged person, while for non-material rights, remorse and seeking forgiveness are sufficient if the wronged person is unaware of them. In the case where an employee was deprived of a deserved raise or promotion, one must seek their forgiveness and pay them the money they were deprived of, and seek help from someone who can intercede for you. If you are unable, increase your remorse and seeking forgiveness.

As for non-material rights, if the wronged person is unaware of your injustice, remorse and seeking forgiveness are sufficient for you. If they are aware, it is necessary to seek their forgiveness, unless you fear a greater harm.