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)

(B)  
What are the essential rules of restitution?

The following, and they are those which natural equity dictates. By restitution, that which another lacks or might lack unfairly is given to him or restored to him. That which is restored must be the thing itself or its exact equivalent, neither more nor less, in so far as the person possessed it previously, whether actually or virtually: further, one must take into account whatever change may have taken place in the thing restored and make proper restitution as the case may be; and further, one must take into account the consequences that are or were prejudicial to the owner of the object due to the detention thereof. An object to be restored must be given back to the owner and not to any other person unless the latter be acting for the owner. He, whosoever he may be, who is in possession of an object to be restored is the one who must make restitution thereof, or whosoever is the one who is responsible for the act which was an offence against justice. Restitution must be made without delay, except in the case when the immediate restoration of an object is impossible (LXII. 2-8).

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