Earlier quoted context omitted.
> You don't have that - the owner first looked up the exchange rate to US dollars What? Any London store that accepts euros will check the rates daily to make sure they can convert back into the GBP price. Currency rates fluctuate all the time, every second of every day. http://uk.finance.yahoo.com/q/bc?s=GBPEUR=X&t=1y&c=
Then they aren't accepting euros as currency. At the end of the day they will immediately convert the euros to GBP. They are willing to use euros as a medium of exchange, but only because they know they can exchange them for (to them) real currency. (Currency is relative, obviously. And just as obviously it helps that somewhere in the world there are people who use that type of money, which gives you comfort that eve…
Any person or business is only willing to use a currency as a medium of exchange because they know they can exchange them for other goods. "Currency" and "medium of exchange" aren't mutually exclusive, the former is a special form of the latter that is recognized widely (and in modern times, usually has no intrinsic value).
> * To be money you have to be willing to save it. If all you do is use it as an intermediate it's not money - yet.*
That's an insane definition for "money." You would claim that the fast food wages of a teenager who always spends it immediately is not money.