I am looking for a countable noun which can stand for either a coin or a banknote. Currency and money are the only two words I can think of, but not being countable they don't fit what I'm looking for, and they can also be interpreted as other forms of payment, such as credit/debit cards.
Open your wallet, find the [coin or banknote] of highest denomination, and write it down.
Does such a word even exist, or should I settle for using piece of currency or simply coin or banknote?