While money may not exist on Earth or the core worlds (or the entire federation) per se, there still are Federation credits for use on non-federation worlds, space stations near independent powers, and for commerce with other independent nations (like the negotiations for the Barzan Wormhole). So maybe like on Ferenginar (if it can convert into Latinum) Qo'noS (if they have money), Romulus, or any other economy. So while he may not of used money (as he only bought from Federation worlds), there still is money. It also can be used for worlds which have not made the transition to a post-scarcity/money environment. And to add to that, the Bank of Bolarus implies that some money is used by the federation. So while you are right in that citizens of the federation probably get stuff for free, that doesn't mean money doesn't necessarily exist (which you did point out.)