Mon

Mons in a physical form: coins (top) and banknotes (bottom).

The mon is an officially used currency in RobiHachi. It is not explicitly stated to be the only used currency across the galaxy, but certainly seems to be such. As per Gras's words, its worth is currently unstable.
Mons predominantly come in an electronic form with payments realized through smartbracelets. However, cash is still in use. While there are no apparent problems with accepting cash, and it is even shown that Yang himself loans money in paper banknotes, this method of paying is suggested to be outdated at the current time, judging by the reaction of a food stall owner when Robby paid him with coins.
Mons were the currency used in Japan during the Edo period when the original Hizakurige story takes place.