Medieval Islamic merchants moved money with the suftaja, a fee-based payment order

Long-distance medieval trade required merchants to move real value across hundreds or thousands of miles of caravan routes vulnerable to theft, shipwreck and simple loss — carrying physical silver from Basra to Samarkand, or across the wider Mediterranean and Islamic world, was a standing risk to any trader's entire capital. What merchants needed was a way to deposit money in one city and draw an equivalent sum in another, without physically transporting it.

Islamic law's prohibition on riba (usury/interest) forbade profiting from the simple lending of money, the same constraint European bankers under Christian usury law faced around the same underlying problem. Any instrument that let a financier earn a return purely from holding or transferring someone else's money risked reading as disguised interest — a serious religious and legal problem, not just a regulatory inconvenience.

Islamic merchants and bankers developed the suftaja, attested in Arabic papyri from the 9th century: a written payment order letting a merchant deposit funds with one banker and have an equivalent sum paid out by a correspondent banker in a distant city.

Used a written payment order between correspondent bankers, same currency at both ends, income limited to a flat upfront fee, so no gain could be mistaken for usury.

The flat fee was disclosed and separate from any currency risk, so it didn't look like interest.

The same-currency structure avoided the exchange-rate spread that could be mistaken for usury.

The fee was for the transfer service itself, a legitimate charge, not for the use of money.

This structure was transparent, so there was nothing hidden for religious authorities to object to.

When a rule blocks your profit mechanism, strip the disputed profit out and charge a flat, disclosed fee for the service itself.

The suftaja became a standard instrument across the medieval Islamic world and was independently adopted by Jewish merchants under the Geonic authorities, who pragmatically ruled in its favor on commercial-necessity grounds even where its convenience arguably resembled tacit interest — one Gaon reasoning that 'because we see that people use this document, we have begun to make judgments based upon it, so that trade not be banished from among the men.'