The Federation credit was a monetary unit used by the United Federation of Planets.
Although it was stated on more than one occasion that the economy of the future was very different, and that money no longer existed on Earth from as early as the late 22nd century or in the Federation as late as the 24th century, this medium of exchange did still exist within that period. All known examples of credit use were via transactions outside or on the periphery of the Federation. (DS9: "In the Cards"; VOY: "Dark Frontier"; Star Trek: First Contact)
Later that same year, Uhura was offered a tribble for ten credits by a bartender on Deep Space Station K-7, a border outpost on the edge of Federation space. She later received one gratis from fellow Federation citizen Cyrano Jones. (TOS: "The Trouble with Tribbles")
Almost a century later, the Federation would have paid the Barzanian Planetary Republic, a non-Federation government, 1.5 million Federation credits as a lump sum and then 100,000 credits every Barzanian year for the rights to the Barzan wormhole. The transaction never transpired, once the wormhole's instability became apparent. (TNG: "The Price", okudagram)