One would assume that the interstellar community would be accustomed to strange barter deals as the primary form of payment, as local currencies would indeed be worthless. If you got a billion Earth dollars from trustworthy Earthlings as payment for your plasma inducer, you could not know the true value of that sum - sailing to Earth with your billion, you might buy a lump of nickel that satisfactorily compensates for your inducer, but you might also find out Earth values its nickel more than your own culture does and you'd only get a disappointingly tiny amount with the billion (even though you thought you had the rate of the Earth dollar down pat after discussing with the Earthlings how much sodium carbonate you can buy with it). Better to barter directly for the nickel, then.
It would be a system like this that would be in place when the United Federation of Planets was founded. Would moving to a single currency be worth the hassle? Or should barter still be encouraged, as it would also encourage individual planets to retain their self-sustenance and keep the UFP much more robust against internal and external threats, financial or otherwise?