Externally the Federation has money to handle aliens. Aliens have stuff, that they want. Therefore trade occurs on a planetary/imperial scale where credits (money) can be used as a stand in for goods to be redeemed at a later date. Individuals inside the federation are provided for completely, they don't have to buy anything because every thing is given to them free of charge. Not only is there no money but there is nothing to buy, so the only place to redeem Federation Credits in the Federation is from the Government it self.
Internally, with %99 unemployment, Kasiday doesn't get special treatment, just because she is the only human in the universe (other than Ralph) who still believes in money. If she wants to be paid, when there are 10 thousand other human Freighter captains who want to do the same job for free, no one is seeing the benefit of keeping Kasiday on the payroll, when you are giving her money that she can't even spend, unless they pay her in alien currency.
After saying all this, there has to be an illegal black market where humans buy and sell "stuff" with some kind of cryptocurrency or alien currency, because the Federation is a bland unadventurous nanny state who won't even put real sugar in ice cream anymore.
Internally, with %99 unemployment, Kasiday doesn't get special treatment, just because she is the only human in the universe (other than Ralph) who still believes in money. If she wants to be paid, when there are 10 thousand other human Freighter captains who want to do the same job for free, no one is seeing the benefit of keeping Kasiday on the payroll, when you are giving her money that she can't even spend, unless they pay her in alien currency.
After saying all this, there has to be an illegal black market where humans buy and sell "stuff" with some kind of cryptocurrency or alien currency, because the Federation is a bland unadventurous nanny state who won't even put real sugar in ice cream anymore.