In a mutuum, or a more complex contract which includes a mutuum such as a home mortgage which allows for a deficiency judgment against the borrower, the borrower’s obligation to repay remains even if the proceeds and all of the things purchased with the proceeds are consumed. We tend to think of ‘loan for consumption’ as referring to what kind of thing is lent or what is done with the proceeds, as opposed to what the contract authorizes and requires. But the distinction between a mutuum and other contracts is not in the kind of property which is exchanged or in what the borrower does with the property; it is in the nature of the agreement itself. A mutuum contemplates and provides for consumption or alienation of the property which is lent in return for a personal IOU. Here is Aquinas again:
As the Philosopher says in the Politics, things can have two uses: one specific and priple, the specific and primary use of shoes is to wear them, and their secondary use is to exchange them for something else. And conversely, the specific and primary use of money is as a means of exchange, since money was instituted for this purpose, and the secondary use of money can be for anything else, for example, as security or for display. And exchange is a use consuming, as it were, the substance of the thing exchanged insofar as the exchange alienates the thing from the one who exchanges it. And so if persons should lend their money to others for use as a means of exchange, which is the specific use of money, and seek a return for this use over and above the principal, this will be contrary to justice. But if persons lend their money to others for another use in which the money is not consumed, there will be the same consideration as regarding the things that are not consumed in their very use, things that are licitly rented and hired out. And the reasoning is the same if a person gives money to another to use it for display, just as, conversely, if one gives shoes to another as a means of exchange and on that account were to seek a recompense over and above the value of the shoes, there would be interest-taking.
Lets assume for the sake of argument that charging interest based on counterfactuals (opportunity cost) is morally licit, even though it isn’t: that mutuum lending for interest is justifiable based on counterfactual storytelling about things that might or might not have happened if different choices were made.
As is typical of modern anti-realist views of property (see Question 10 for a realist view), this gets things almost exactly backwards. In fact if the argument from counterfactuals or opportunity cost were valid in the first place, what would follow is that the lender should pay interest to the borrower.
Suppose you lend me fresh peaches, and I personally guarantee to give you the same number of fresh peaches six months from now.
In order to provide you with fresh peaches six months from now I have to take risks and invest more capital and labor. If I just hang on to your peaches and return them to you they will be rotten, because the peaches you lent to me are subject to decay. You should pay me interest, since when I give you fresh peaches in six months you are getting a greater value back than what you gave. I personally guaranteed you fresh peaches in six months, and took all of the risk and labor of providing them upon myself. (Note: Question 46 provides a description of a non-usurious futures contract, that is, a futures contract for profit which is not based on a mutuum loan).