How can one advise a person who prefers retribution against those who harmed him in the Hereafter over forgiving them and receiving multiplied reward in this world, justifying this by saying that it harms whoever harmed him and takes from their good deeds, even if they are few?

Forgiveness is something that is encouraged and desired in Islamic law; it is not obligatory. It is permissible for the wronged person to seek retribution from the oppressor without exceeding the bounds. If he does not seek retribution in this world, it will be sought for him from the oppressor in the Hereafter. Therefore, the brother should be reminded of the virtues of forgiveness and encouraged to emulate the conduct of the righteous predecessors in overlooking offenses.