Objection 1. It would seem that a man is bound to restore whatever profits he has made out of money gotten by usury. For the Apostle says (Romans 11:16): "If the root be holy, so are the branches." Therefore likewise if the root be rotten so are the branches. But the root was infected with usury. Therefore whatever profit is made therefrom is infected with usury. Therefore he is bound to restore it.
Objection 2. Further, it is laid down (Extra, De Usuris, in the Decretal: 'Cum tu sicut asseris'): "Property accruing from usury must be sold, and the price repaid to the persons from whom the usury was extorted." Therefore, likewise, whatever else is acquired from usurious money must be restored.
Objection 3. Further, that which a man buys with the proceeds of usury is due to him by reason of the money he paid for it. Therefore he has no more right to the thing purchased than to the money he paid. But he was bound to restore the money gained through usury. Therefore he is also bound to restore what he acquired with it.
On the contrary, A man may lawfully hold what he has lawfully acquired. Now that which is acquired by the proceeds of usury is sometimes lawfully acquired. Therefore it may be lawfully retained.
I answer that, As stated above (Article 1), there are certain things whose use is their consumption, and which do not admit of usufruct, according to law (ibid., ad 3). Wherefore if such like things be extorted by means of usury, for instance money, wheat, wine and so forth, the lender is not bound to restore more than he received (since what is acquired by such things is the fruit not of the thing but of human industry), unless indeed the other party by losing some of his own goods be injured through the lender retaining them: for then he is bound to make good the loss.
On the other hand, there are certain things whose use is not their consumption: such things admit of usufruct, for instance house or land property and so forth. Wherefore if a man has by usury extorted from another his house or land, he is bound to restore not only the house or land but also the fruits accruing to him therefrom, since they are the fruits of things owned by another man and consequently are due to him.
Reply to Objection 1. The root has not only the character of matter, as money made by usury has; but has also somewhat the character of an active cause, in so far as it administers nourishment. Hence the comparison fails.
Reply to Objection 2. Further, Property acquired from usury does not belong to the person who paid usury, but to the person who bought it. Yet he that paid usury has a certain claim on that property just as he has on the other goods of the usurer. Hence it is not prescribed that such property should be assigned to the persons who paid usury, since the property is perhaps worth more than what they paid in usury, but it is commanded that the property be sold, and the price be restored, of course according to the amount taken in usury.
Reply to Objection 3. The proceeds of money taken in usury are due to the person who acquired them not by reason of the usurious money as instrumental cause, but on account of his own industry as principal cause. Wherefore he has more right to the goods acquired with usurious money than to the usurious money itself.
Source: Summa Theologica (New Advent)