You can buy the wine but I’ll keep the refreshment

A usurer attempts to sell some actual property to a “borrower” while retaining the potential for profit which inheres in that actual property.

The borrower in a mutuum owns the actual property he is “lent”, because the mutuum authorizes him to dispose of or consume that actual property as he sees fit: his obligation to the lender is a personal obligation, independent of what happens to that actual property.  Once a mutuum has been joined the borrower fully owns the actual property and the lender owns nothing at all: the lender merely has the borrower’s personal pledge of some different unspecified property of the same worth, at some time in the future.

But any potential which inheres in actual property is not separable from that actual property.  You can’t sell that wine while retaining that wine’s potential for refreshment.  So another way to think about usury is that it represents an attempt to sell property without really selling it.  The usurer transfers full ownership of the property to the borrower personally, since the borrower’s obligation under the agreement remains however he disposes of that actual property; and simultaneously the usurer insists on guaranteed actual profits (not even just potential profits!) from property he no longer owns.

It is fair to ask, then, if any profits from a mutuum are immoral why would anyone lend under a mutuum?  And the answer is that we should never, categorically, lend under a mutuum in pursuit of financial self interest: mutuum lending is only morally licit as an act of charity or friendship.