Debt law is simple. All debt is to the Pharaoh. Private debt is illegal. There are no money lenders in the Last City. There are Safe Houses, but they are not banks. They are merely vaults that people may pay to have their possessions protected.
If someone commits a crime against the Pharaoh they may be fined. Those that cannot pay become debters, commonly called the Collared.
The Collared are called such because of the collars that they wear. Each collar has a length of chain attached to it that represents the debt they owe. Different types of links represent different amounts. The links are made of a lightweight metal that no steel can mark.
Anyone my "purchase a link", which entails going to a Vizir, describing a task they need done, paying a sum of money and receiving a writ which can be delivered to any debtor (or debtors).
The debtor cannot refuse a writ. They must take the contract. If they fail the task, or refuse it, their debt may actually be increased (there are many contributing factors to what happens in the event of a contract failure. generally, they are all best avoided).
Once the last link bought and the last contract complete, the collar is removed.
There are a small number of people, marked by gold collars, who have no debt but still take contracts. Instead of working off links, they earn them and use them as currency and prestige. These people are called the Golden Servants. Many of them are former Debtors.