Objection 1. It would seem that it is not sufficient to restore the exact amount taken. For it is written (Exodus 22:1): "If a man shall steal an ox or a sheep and kill or sell it, he shall restore five oxen for one ox, and four sheep for one sheep." Now everyone is bound to keep the commandments of the Divine law. Therefore a thief is bound to restore four- or fivefold.
Objection 2. Further, "What things soever were written, were written for our learning" (Romans 15:4). Now Zachaeus said (Luke 19:8) to our Lord: "If I have wronged any man of any thing, I restore him fourfold." Therefore a man is bound to restore several times over the amount he has taken unjustly.
Objection 3. Further, no one can be justly deprived of what he is not bound to give. Now a judge justly deprives a thief of more than the amount of his theft, under the head of damages. Therefore a man is bound to pay it, and consequently it is not sufficient to restore the exact amount.
On the contrary, Restitution re-establishes equality where an unjust taking has caused inequality. Now equality is restored by repaying the exact amount taken. Therefore there is no obligation to restore more than the exact amount taken.
I answer that, When a man takes another's thing unjustly, two things must be considered. One is the inequality on the part of the thing, which inequality is sometimes void of injustice, as is the case in loans. The other is the sin of injustice, which is consistent with equality on the part of the thing, as when a person intends to use violence but fails.
As regards the first, the remedy is applied by making restitution, since thereby equality is re-established; and for this it is enough that a man restore just so much as he has belonging to another. But as regards the sin, the remedy is applied by punishment, the infliction of which belongs to the judge: and so, until a man is condemned by the judge, he is not bound to restore more than he took, but when once he is condemned, he is bound to pay the penalty.
Hence it is clear how to answer the First Objection: because this law fixes the punishment to be inflicted by the judge. Nor is this commandment to be kept now, because since the coming of Christ no man is bound to keep the judicial precepts, as stated above (I-II, 104, 3). Nevertheless the same might be determined by human law, and then the same answer would apply.
Reply to Objection 2. Zachaeus said this being willing to do more than he was bound to do; hence he had said already: "Behold... the half of my goods I give to the poor."
Reply to Objection 3. By condemning the man justly, the judge can exact more by way of damages; and yet this was not due before the sentence.
Source: Summa Theologica (New Advent)