It's actually more nuanced than that. Interest isn't banned outright. Interests on things that carried risk was fine so interest on foreign exchange was fine. This was how the banking families of Italy like the Medici made so much money. The ban on usury is only on non-secured loans or loans secured by the person. For example, mortgage is actually OK because the debtor can default and walk away. However, loans that are secured by the person who must pay cannot collect interest. This, IIRC, comes from St. Thomas Aquinas.