This was my take on why Spock didn't understand "exact change." If the request was for "exact monitary amount for fare" or something similar, then Spock likely wouldn't have been confused.
I certainly don't take Spock confusion as a indicator that the 23rd (or 24th) century don't engage in monitary value exchanges for purchaces.
No physical money though? No problem.
Kirk says in TVH: "They're still using money, we'll need to get some."
Not "they're still using physical currency, we'll need to get some," and it doesn't make sense to interpret it in that way, either. Even if there was only virtual currency/credits/whatever they'd still need to get some
. "They're still using money" there only makes sense if Kirk et al come from a time where they don't use money, of any description.
Lily asks Picard "How much did this thing cost?" Picard responds "The economics of the future are somewhat different. You see, money doesn't exist in the 24th century."
It again makes no sense for Picard to offer that as a response unless he is saying not that physical currency is obsolete but that money itself is no longer in use.
Plenty of contradictions, of course. Possibly the only aspect of Trek where canon is more
irretrievably self-contradictory is the precise workings of the transporter.