They did it a couple of ways as far as I know. The first, and easiest to explain, is that they ignored usury laws or changed the meaning of usury to mean excessive interest charges. The second was to charge a fee on any loan that they gave out. The fee was deemed legal since it was basically covering the loss that the bank would have taken by not having that money to invest. There was also a gentleman's agreement that the loans could comewith a penalty for late repayment and the borrower would intentionally pay late.

What is interesting is that, at times, the Church itself acted as a bank and charged interest on its loans to states and private citizens.

The Council of 1917 does not have an explicit mention of usury being a single, so now it is perfectly fine in the Churches eye.