Trading and letters of credit
Step back in history to a time when merchants traveled across continents to buy cloth in one country to sell in another country. As a Florentine wool merchant, you might make a journey to Amsterdam to buy fine wool in that newly formed city-state, whose port collected resources from the whole of Northern Europe and beyond. You could then transport the wool to Florence, where it could be sold to tailors making fine garments for their wealthy clients. We're talking about 1300 AD—a time when it was not safe to carry gold or other precious metals as a form of currency to buy and sell goods. What was necessary was a form of currency that worked across country boundaries, one that could be used in Amsterdam and Florence, or anywhere!
Marco Polo had been to China and had seen how commerce was conducted in that thriving economy. At the heart of the successful Khan empire were advanced financial techniques that we would recognize today. Fiat currencies...