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. 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. Also the existence of the bank of Bolarus seems to imply there is a currency for the federation (credits).