Luigi Di Maio, an Italian deputy prime minister, took aim at President Emmanuel Macron of France this week, laying the blame for migration, a deeply divisive issue in Italy, on Mr. Macron.

Citing a French-backed currency known as the C.F.A. franc that is used by 14 nations in western and central Africa, Mr. Di Maio accused France of using the currency to exploit former colonies.

Mr. Di Maio, a member of the populist Five Star movement, which is skeptical of the European Union and Mr. Macron’s more internationalist views, set off the diplomatic spat with France when he said that Mr. Macron “first lectures us, then continues to finance public debt with the money with which he exploits Africa,” according to ANSA, the Italian press agency.

But the currency, its history and the issues around it have had little to do with the debate over African migration to Europe until now. So what is the C.F.A. franc and how did it come to be at the center of diplomatic discord?