There are plenty of contradictions in Star Trek. Sometimes it's a matter of writing error that slips past editing and manages to get approved... and even the actors don't stop to consider it. Take the whole thing with Quark and gold
. At one point he says it's worthless, and at another point he says otherwise.
"No money in the 24th century" is just a joke, IMHO. Now, it could be extended a bit... "We don't have money--we have a closed credit system." But how VALUABLE are credits anyway? To me, it looks like when you play within the confines of the Federation, you don't really need much. It's a socialist society. Healthcare is free. Food is essentially free. Replicator "rations" or credits may be dependent upon the quantity or complexity of what you're requesting to have produced, but we're not shown how many credits are needed for whatever thresholds that might be crossed. There's probably a huge apartment system managed by the government that enables basic lodging for free.
But how do you obtain exclusive property? There's always something exclusive or of limited availability in society. Are you bequeathed that by your station or by making some special achievement? How much does the government really own and/or control? If replicators can produce precious metals, then the whole monetary system can't work. People would just replicate platinum and then sell it.
Anyway, this is all within the confines of the Federation. Externally, it's clear that most species have some kind of monetary system and so in order to trade you MUST have some kind of exchange vehicle. Can you trade your Federation credits for something you want to buy from a Ferengi? Maybe you have to get your credits exchanged to latinum first.
Bottom line... Star Trek did a poor job of really explaining how the Federation credit system is supposed to work and how it would interface with extraterrestrial cultures. We've heard inferences about crew members having salary here and there, but we've no idea how much and how it positions them economically. While I know it could've been a powder keg trying to define a monetary system in more detail, I think it would've been possible to do something a little more detailed than what we got.