Objection 1. It would seem that it is not necessary to restore what has been taken away. For that which is impossible is not necessary for salvation.
But sometimes it is impossible to restore what has been taken, as when a man has taken limb or life. Therefore it does not seem necessary for salvation to restore what one has taken from another.
Objection 2. Further, the commission of a sin is not necessary for salvation, for then a man would be in a dilemma. But sometimes it is impossible, without sin, to restore what has been taken, as when one has taken away another's good name by telling the truth. Therefore it is not necessary for salvation to restore what one has taken from another.
Objection 3. Further, what is done cannot be undone. Now sometimes a man loses his personal honor by being unjustly insulted. Therefore that which has been taken from him cannot be restored to him: so that it is not necessary for salvation to restore what one has taken.
Objection 4. Further, to prevent a person from obtaining a good thing is seemingly the same as to take it away from him, since "to lack little is almost the same as to lack nothing at all," as the Philosopher says (Phys. ii, 5). Now when anyone prevents a man from obtaining a benefice or the like, seemingly he is not bound to restore the benefice, since this would be sometimes impossible. Therefore it is not necessary for salvation to restore what one has taken.
On the contrary, Augustine says (Ep. ad Maced. cxliii): "Unless a man restore what he has purloined, his sin is not forgiven."
I answer that, Restitution as stated above (Article 1) is an act of commutative justice, and this demands a certain equality. Wherefore restitution denotes the return of the thing unjustly taken; since it is by giving it back that equality is reestablished. If, however, it be taken away justly, there will be equality, and so there will be no need for restitution, for justice consists in equality. Since therefore the safeguarding of justice is necessary for salvation, it follows that it is necessary for salvation to restore what has been taken unjustly.
Reply to Objection 1. When it is impossible to repay the equivalent, it suffices to repay what one can, as in the case of honor due to God and our parents, as the Philosopher states (Ethic. viii, 14). Wherefore when that which has been taken cannot be restored in equivalent, compensation should be made as far as possible: for instance if one man has deprived another of a limb, he must make compensation either in money or in honor, the condition of either party being duly considered according to the judgment of a good man.
Reply to Objection 2. There are three ways in which one may take away another's good name. First, by saying what is true, and this justly, as when a man reveals another's sin, while observing the right order of so doing, and then he is not bound to restitution. Secondly, by saying what is untrue and unjustly, and then he is bound to restore that man's good name, by confessing that he told an untruth. Thirdly, by saying what is true, but unjustly, as when a man reveals another's sin contrarily to the right order of so doing, and then he is bound to restore his good name as far as he can, and yet without telling an untruth; for instance by saying that he spoke ill, or that he defamed him unjustly; or if he be unable to restore his good name, he must compensate him otherwise, the same as in other cases, as stated above (ad 1).
Reply to Objection 3. The action of the man who has defamed another cannot be undone, but it is possible, by showing him deference, to undo its effect, viz. the lowering of the other man's personal dignity in the opinion of other men.
Reply to Objection 4. There are several ways of preventing a man from obtaining a benefice. First, justly: for instance, if having in view the honor of God or the good of the Church, one procures its being conferred on a more worthy subject, and then there is no obligation whatever to make restitution or compensation. Secondly, unjustly, if the intention is to injure the person whom one hinders, through hatred, revenge or the like. In this case, if before the benefice has been definitely assigned to anyone, one prevents its being conferred on a worthy subject by counseling that it be not conferred on him, one is bound to make some compensation, after taking account of the circumstances of persons and things according to the judgment of a prudent person: but one is not bound in equivalent, because that man had not obtained the benefice and might have been prevented in many ways from obtaining it. If, on the other hand, the benefice had already been assigned to a certain person, and someone, for some undue cause procures its revocation, it is the same as though he had deprived a man of what he already possessed, and consequently he would be bound to compensation in equivalent, in proportion, however, to his means.
Source: Summa Theologica (New Advent)