Objection 1. It would seem that it is not lawful to borrow money under a condition of usury. For the Apostle says (Romans 1:32) that they "are worthy of death... not only they that do" these sins, "but they also that consent to them that do them." Now he that borrows money under a condition of usury consents in the sin of the usurer, and gives him an occasion of sin. Therefore he sins also.
Objection 2. Further, for no temporal advantage ought one to give another an occasion of committing a sin: for this pertains to active scandal, which is always sinful, as stated above (Question 43, Article 2). Now he that seeks to borrow from a usurer gives him an occasion of sin. Therefore he is not to be excused on account of any temporal advantage.
Objection 3. Further, it seems no less necessary sometimes to deposit one's money with a usurer than to borrow from him. Now it seems altogether unlawful to deposit one's money with a usurer, even as it would be unlawful to deposit one's sword with a madman, a maiden with a libertine, or food with a glutton. Neither therefore is it lawful to borrow from a usurer.
On the contrary, He that suffers injury does not sin, according to the Philosopher (Ethic. v, 11), wherefore justice is not a mean between two vices, as stated in the same book (ch. 5). Now a usurer sins by doing an injury to the person who borrows from him under a condition of usury. Therefore he that accepts a loan under a condition of usury does not sin.
I answer that, It is by no means lawful to induce a man to sin, yet it is lawful to make use of another's sin for a good end, since even God uses all sin for some good, since He draws some good from every evil as stated in the Enchiridion (xi). Hence when Publicola asked whether it were lawful to make use of an oath taken by a man swearing by false gods (which is a manifest sin, for he gives Divine honor to them) Augustine (Ep. xlvii) answered that he who uses, not for a bad but for a good purpose, the oath of a man that swears by false gods, is a party, not to his sin of swearing by demons, but to his good compact whereby he kept his word. If however he were to induce him to swear by false gods, he would sin.
Accordingly we must also answer to the question in point that it is by no means lawful to induce a man to lend under a condition of usury: yet it is lawful to borrow for usury from a man who is ready to do so and is a usurer by profession; provided the borrower have a good end in view, such as the relief of his own or another's need. Thus too it is lawful for a man who has fallen among thieves to point out his property to them (which they sin in taking) in order to save his life, after the example of the ten men who said to Ismahel (Jeremiah 41:8): "Kill us not: for we have stores in the field."
Reply to Objection 1. He who borrows for usury does not consent to the usurer's sin but makes use of it. Nor is it the usurer's acceptance of usury that pleases him, but his lending, which is good.
Reply to Objection 2. He who borrows for usury gives the usurer an occasion, not for taking usury, but for lending; it is the usurer who finds an occasion of sin in the malice of his heart. Hence there is passive scandal on his part, while there is no active scandal on the part of the person who seeks to borrow. Nor is this passive scandal a reason why the other person should desist from borrowing if he is in need, since this passive scandal arises not from weakness or ignorance but from malice.
Reply to Objection 3. If one were to entrust one's money to a usurer lacking other means of practising usury; or with the intention of making a greater profit from his money by reason of the usury, one would be giving a sinner matter for sin, so that one would be a participator in his guilt. If, on the other hand, the usurer to whom one entrusts one's money has other means of practising usury, there is no sin in entrusting it to him that it may be in safer keeping, since this is to use a sinner for a good purpose.
Source: Summa Theologica (New Advent)