I don't know the answers to those questions (I think I read somewhere that, in principle, fiat money is ok. Not sure though). However, the thing to keep in mind is that in the real world there is no such thing as fiat money that is issued by the government. All (most?) currencies are actually bank notes (for example, if you look at US currency, it does not say, "US Treasury note" or "US Govt Note". Instead, it says, "Federal Reserve Note" (the Fed is an amalgamation of private banks). That is, (very basically speaking) they are loans by the banks to the government, and interest is owed on those notes. Also consider the fact that banks and governments collude to such an extent that they are essentially one and the same...I don't know what that means for the shariah status of such currencies. I hate banks with a passion. May Allah destroy them all.