The franc was a unit of money found in a handful of European nation-states, including France, as well as several African nation-states.
In the holosuite program Julian Bashir, Secret Agent, Bashir as Patrick Merriweather had to come up with 5,000,000 francs in order to have a meeting arranged between himself and Dr. Noah. Merriweather was able to come up with the money after winning it from Mr. Duchamps in the card game baccarat. (DS9: "Our Man Bashir")
The holographic Le Coeur de Lion earned 1,247 francs and 81 Reichsmark one Saturday night, which was, according to Mademoiselle de Neuf, insufficient. (VOY: "The Killing Game")