The thing is we almost NEVER see anyone using currency in Star Trek.
The Ferengi don't have money, they trade with latinum. They don't use credits or paper representing latinum - they use actual bars/strips/slips of latinum.
Nearly all trade is one commodity exchanged for another. The only real currency portrayed has been credits - such as what Uhura used to buy her tribble. I would guess that those credits have a value linked to a real physical commodity and are not cash as we think of it.