What is the proper act of commutative justice?
It is called restitution (LXII. 1).
What is restitution?
It is that act whereby the right relation between one man and another is re-established, whenever this relation has been broken by the fact that one of the two has not received his due (LXI. 1).
Restitution then does not always imply an act of injustice?
No, for also the virtuous man may make restitution, by restoring what belongs to another immediately and with scrupulousness when such thing ought to be restored.
Next - The Catechism of the Summa - SECOND SECTION - A DETAILED SURVEY OF MAN'S RETURN TO GOD - XX. OF THE ACT OF COMMUTATIVE JUSTICE, VIZ., OF RESTITUTION (B)