I've been watching Medici: Masters of Florence on Netflix (pretty fun but not very historical) and the common criticism of the Medici family by their enemies was that they were morally corrupt usurers, which of course the Medici deny in order to maintain favor with the Catholic church.
Now the way I understand usury is that today's definition is lending money at unreasonably high rates of interest, but originally usury meant charging interest of any kind.
I could understand how the bank could make money off the money they were holding for other people by investing it in land and businesses, but if the Medici bank wasn't charging interest, how were they making money from lending? Were they lending at a loss and depending on the profit from the investing side? Or were they secretly charging interest and were lying about not being usurers?
A combination of the prohibition on usury wasn't as strict as commonly portrayed and that there were ways of getting around prohibitions.
Thomas Aquinas resolved in his work that there was a difference between investment and usury. Namely that investment involved risk and it was acceptable whereas usury is when you "know" that you were going to get your money back. In effect this meant that secured loans was usury while unsecured commercial loans wasn't (as you are getting paid for taking a risk). By the mid 14th century the Catholic Church had accepted Aquinas's definition of usury and thus opened the door to commercial lending through banks.
There were also ways of getting around the definition of "usury". One example would be "the exchange of bills", in which there would be negotiations between merchants to "exchange" currency in two different time and places. Often, the currencies in question would be foreign currencies. Thus, they are superficially merely negotiating future rates of conversion but in reality it would be quite easy to hide interest payments in the conversion rates. Religious scholars however tended to accept the merchant's argument that this could not be considered usury.
Another example was through the usage of "late fees", in which if one paid the loan on time, you got the loan for free, however if one were to be late by a couple of days, then the debtor would be 'fined" a certain amount. However: it is also known that should the debtor pay the loan on time, he would have difficulty getting anyone else to loan to him.
Yet another example would be "selling of goods" in which one party would "buy" a bunch of goods at a certain price and store said goods. Later on, after a certain period of time, the party holding the goods would sell those goods back to the original seller, at a price above what the initial transaction what. The difference was, of course, de facto interest payments. But buying goods at one price and selling it another was not considered usury per say, at least superficially.
Usury laws during the Middle Ages and Renaissance certainly constrained lending and interest payments on loans, however, they were not fool-proof and there were very inventive bookkeeping ways of getting around it by calling it something else. This is also not to say authorities and the Church did not try to crack down on inventive means of charging interest. It is better to view usury laws as a constraining rather than forbidding interest payments charged by institutions of finance.
Source: The Rise and Decline of the Medici Bank: 1397-1494 By Raymond De Roover
Wealth and the Will of God: Discerning the Use of Riches in the Service of Ultimate Purpose by Paul G. Schervish, Albert Keith Whitaker
Capitalism: A Modern Economic History By Edmund Clingan