Usury is a sin. Canon 29 of Council of Vienne (I have not seen its bulls on the Vatican.ca site by the way).
What was considered usury? Immoderate interest? Interest taken from the poor? No, those were taken as aggravating circumstances of usury, but usury itself was - lending one sum of money and getting a really higher sum back because of the fact of the loan.
I say really higher sum, because if you lend in Euros and get back in Swedish Crowns, you must have about ten times as many of them, since that is the relative worth of these. If a devaluation has taken place relative to the gold standard or something, and this or that currency is only 9/10 of the gold it was worth at time of loan, obviously the payment must be 10/9 of the nominal sum at the time of paying.
And I say: because of the fact of the loan, because there are titles such as lucrum cessans or damnum emergens.
Lucrum cessans is missing a good opportunity, not because of loan as such, but because of late payment.
In fourteen months time I want to buy a house that will be extra cheap since I book in advance, thus facilitating for the builder to get his loans. I set about to seek a borrower to take care of the money for me so I do not spend it meanwhile. A month later we agree he pays back same sum after a year, one month before option of buying the house. He pays back two months too late for our agreement, one month too late for me to buy the house. I have to buy a more expensive one, or one that costs same or less but which is not as big. An opportunity was missed due to late payment. That is called lucrum cessans.
Annexed question: if it happens I find a house that is really better, both worth more and me paying less, do I still have a claim on borrower paying more due to lucrum cessans? I think it would at least be morally doubtful to charge for lucrum cessans when a better lucrum is had instead.
Damnum emergens is losing something, not just missing something.
I need to pay a transport for my fresh wine a year from now, to a market where I can sell it. The money from this years sale I lend to someone else, but he is three months late, and as the wine was weak (as is the case with some wines not stored for long, like Beaujoulais or Heurigen) and I got no transport, half was not sold in time and a third of that turned into vinegar of a cheaper quality than the wine. So, through negligence of borrower I end up with less worth of wine or wine sales, and vinegar which is worth less. Does he owe me for the three months?
Not for the time as such, but for damnum emergens.
Annexed question: if instead of producing vinegar I get a second fermentation leading to invention of Champagne, I am perhaps not entitled to damage for loss?
Bibl. Heure Joyeuse, Paris
18 - VI - 2011
PS, I am personally not lending money to someone, only considering what in case of loans the thomistic terms lucrum cessans and damnum emergens actually mean.