A buyer of products or services who is able to pay but refuses to pay when those products or services have been received has committed an act of theft or fraud. A buyer who is not able to pay but pretends to be able is likewise committing theft or fraud.
A trickier case is when a merchant extends credit and allows a buyer to pay later. If the ‘buyer’ is an institution and the security on commercial credit is the balance sheet of an institution, this is not a mutuum loan so the usury prohibition does not apply. 
If the buyer is an individual who is personally guaranteeing payment to the merchant, this is a mutuum loan and the prohibition of usury does apply. This gives rise to two possible cases. In one case the buyer is able to pay on time but refuses. In the other case the buyer has suffered some catastrophe and is unable to pay. If the merchant does not have proper security in place then he should absorb the loss until the buyer is able to pay, and should not insist on any penalty above the amount owed. If the merchant does not want to be exposed to those kinds of losses he can arrange for some kind of security (claims against specified property) or he can require payment on delivery instead of extending credit.
The former is theft or fraud; the latter is business misfortune, a risk associated with doing business
My own tentative view is that theft and fraud should generally involve criminal conviction and penalties of some sort, not merely compensation of the victim at the level of tort, because theft and fraud harm the common good not just the victim. One way they harm the common good is by opening the door to various kinds of ‘hidden usury’ – thief and ‘victim’ in collusion attempt to get around the prohibition of usury, by creating a situation in which the borrower ‘defrauds’ the lender, wink wink, so the borrower owes a penalty in addition to the principal. Collusion in faux-theft in order to produce penalties under the legal system – hidden usury – would simply make both parties guilty.
If this seems severe, consider that stealing a pack of gum is criminal theft, because stealing harms not just the victim but the common good. Categorizing it as criminal does not really say anything about the severity of the offense in a specific case; it merely acknowledges the harm to the common good in addition to the victim, or the defrauding of the sovereign by the parties in collusion. A discussion of crime versus tort is beyond the scope of the present FAQ, but it is sufficient to point out that ‘hidden usury’ in this kind of case involves (assuming a just legal system which declines to enforce usurious contracts) a conspiracy between borrower and lender to falsify an act of fraud so that the legal system will enforce a penalty.
50) John Noonan and other scholars have stated that we can’t grasp the usury doctrine without getting into medieval just price theory. Yet you say that usury doctrine doesn’t depend upon any economic theory or theory of just pricing. Why do some scholars say that there is a dependence between usury doctrine and medieval theory of just price?
But a borrower’s promise to repay principal which has been consumed is not property
The way to figure out whether a contract for gain is usurious or not is to look for contract terms which treat a personal guarantee as if it were property. It is morally licit for an owner to profit from the use of his property, or of property against which he has claims. A mere promise of apples is not itself actually apples. And the historical fact that there used to exist some apples which were consumed or money which was spent is not – the historical fact is not – actual apples or money.