According to this question, Maasar Sheni cannot be redeemed using paper money. That certainly seems to be the common assumption, but is that discussed anywhere? Is paper money the same as banknotes, when the money is not used to be traded in for precious metals at the bank anymore?
In what other areas of halakha is our modern from of currency not considered money? Can one use it for Kiddushin (as money, not hanaah/shaveh kesef)? For kinyan kesef (to acquire land)?
What's the reason why or why not paper money, which isn't backed by silver or gold, would be or would not be considered money?