My sources are at work and here it's the weekend. If my answer is still the best come Monday I'll update then.

A common practice was two deeds of sale for a small item of nominal value.

Example: you (borrower) enter into two contracts with me (lender). In the first, I will pay you $100 (loan amount) for your spoon (nominal item). In one year you will under the second contact buy back the nominal item from me for $110 (loan amount plus "interest").

If you fail to buy the item back per the contact, you are a debtor you can be placed into slavery until your debt is repaid. This firm of slavery is distinguished from common understandings of the word. The debtor still has nearly full suite of social and civil rights. Any unsure harshness from me over you while your remain in slavery to me will negate the debt.

This is TL;DR answer, there are many more subtleties. However I suspect no one want a full text-wall.

My authority to answer: LL.M in finance with research in comparative traditions of inter alia Arabic and Persian financial arrangements.

Ask questions if you want more details.