In order to understand usury, we need to have a clear understanding of several concepts that provide the foundation of the moral understanding of usury. Here I am not attempting a complete and comprehensive articulation of these concepts, but simply a sufficient explanation to understand usury.
The most fundamental concept necessary for understanding usury is justice in exchange, which is a species of communitive justice. Justice in exchange is minimally determined by an equality established between the terms of the exchange, that is the things that are bought and sold are worth proportionately the same amount. For the sake of understanding usury, it is not necessary to have a complete theory of justice price, but rather accepting the tautology that two instances of the same kind of property possess the same value, e.g. 2 apples is worth 2 apples. The problem of just price becomes relevant when addressing the question of how many oranges 2 apples is worth.
The next is to understand some of the aspects of property that make it property. The most important for our discussion is that property is something that can be alienated from the owner. More simply it is something that can be exchanged, which involves being alienated from the owner. This means that things like feelings, thoughts, time and such things cannot be alienated and are thus not property.
Finally, it is important to understand some concepts about contracts, because as Benedict XV teaches the sin lies in the mutuum contract and that contracts differ one from another.
A contract of exchange is defined materially by the property or services that are exchanged. That is there are many formally different contracts, e.g. sale vs rent, which involve the same property. It is important to understand the formal aspect of the contract which also defines how the property is to be considered or treated. A contract that treats a silver cup as money or as a cup differ formally with respect to the terms that define the contract in the manner of how the property is considered.
The mutuum contract from which the son of usury may arise is a particular type of contract. In the tradition of Roman Law it is called a contract for consumption. However, this consumption taken broadly means any form of alienation from the owner and thus includes exchanged the thing for something else. It is in this sense that it is consumed. As noted above since property as such includes the possibility of alienation through exchange as well as other ways such as destruction with respect to material property, any property can be the subject of usury and not simply money. Thus it is in this sense that the mutuum is define where the property is considered under the aspect of consumption or alienation.
Moreover, the mutuum is defined by the other term of the contract, that is what is given for the property received under the aspect of consumption. What is given is a personal promise of return in kind. Since the property is considered under the aspect of consumption, the thing itself is not returned but something in kind.
The question of usury then arises as to whether a mutuum contract can be entered for profit or if one can charge for more than what was given. Given communitive justice mentioned above, the borrower owes at least owes a return of the principle in the same amount but does he owe more?
The answer is no and charging more is the sin of usury.
The logic is whence arise the ability charge more. Since the property is considered under the aspect of consumption ownership is transfer and when the thing is consumed there is nothing remaining to be charged for. Thus charging more is a sort of double charge where the thing is charged more than its worth (e.g. two apples is charged for one apple) or there is charge for what does not exist. Both are manifestly unjust in violating communitive justice.
There are many other circumstances that are similar to usury is a mutuum contract, such as a corporate bond or insurance contract which generally differ formally from the mutuum when considered in themselves. There are many objections to the doctrine of usury that involve false premises or fallacious reasoning. However for this post I’ve simply attempted to spell out the usury doctrine simply and clearly. Zippy has dealt with many objections in his book and blog and has a manifestly better presentation.