The same incentive you'd have to do something for someone without receiving payment - a desire to help. I'd imagine the fact that people in that time had stronger familial ties to others in their village also helped increase payback rates.
And both religious created a way to indirectly pay interest by turning it into a business partnership.
i.e. when you are helping someone (a friend, relative) you don't pay interest. But when it's a business loaning the money that desire to help doesn't exist, and it becomes a business partnership instead.
You're supposed to use Mudarabah[0] preferentially, which is dramatically less of a lazy hack around usury rules than Murabaha. (Also as a English speaker, those words are very hard to distinguish.)
Assuming such a rule would mean "all mandatory fees are prohibited" and that we're talking about profit driven ventures (not cooperatives or patrons that might have other reasons to want to lend money than monetary profits), you can be creative and charge money in ways that technically doesn't count as interest.
The Medici bank gained lots of their money from foreign exchange speculation issuing a sort of option for buying foreign currencies at a specific date (this would probably be a lot harder to do today), you could also charge late fees with the social understanding that paying a loan back on time is really bad for your credit score. I guess a modern conglomerate might make money by putting certain terms in effect: You want to build a house but need some money to do so? I will lend you the money if you contract my contractors to build your house. A university could lend you the money but keep charging extortionate tuitions as long as the cost for the education service is unregulated.
One of the major dangers with interest is it compounds in bad times. Someone gets laid off for 6 months and suddenly owes more money, this is especially problematic if someone gets injured and can no longer make the same income etc.
Fees are one option. If you agree to a 2 year loan of 100$ as long as they pay you back 110$ that’s acceptable, the difference being if it takes them 3 years you still only get 110$. Obviously this runs into problems with inflation and people skipping payments etc, but it’s one socially “fair” way to handle loans between friends and family.
Well if you're a credit union that operates in Shithole Town, USA... then in theory if you lend money to people who improve the community, you'll reap the benefits everyone else does by living in a more desirable place (more valuable property, more customers for businesses, etc).
Almost impossible to imagine given the current state of the economy where every possible thing you can imagine has been commodified.
