A economic thing, that is something with utility that is exchanged, can be considered in two respect: with respect to the thing itself and with respect to its use.
In Aquinas’ argument against usury, he talks about two different injustices with respect to usury: charging for what does not exist or double charging. The economic thing Aquinas considers is something that is consumed in its use, such that the use of a thing and its being cannot be separated. We can then consider charging interest or rent on this thing in the two ways above.
In charging for the thing considered in itself, we are double charging. We are charging for the thing itself twice, because we charge first for the thing, which must be returned and thus is of equal value, and in addition we are charging rent for use. However, rent for use is not separate from the thing itself. Hence, this constitutes a double charge, that is a charge for the thing twice, which is manifestly unjust.
In charging for the thing considered in its use, we charge for something that does not exist. First the usurer charges for the thing itself and this is expected as a return. However, in addition, he charges for the use, but in the use the thing ceases to exist. Hence, the usurer is charging for something that no longer exist which has already been charged for.
Thus, the two ways of considering the injustice of usury are equivalent in so far as they concern something actually inseparable, the use and being of the thing, while logically distinct in the consideration of the thing itself and the use of the thing.