Actually cash is a form of that: a $100 bill is a bank note that you have $100 in the bank. In theory if you want to deposit $1 million, the bank should just give you a note for a $1 million deposit - that would be a $1 million bank note. But all this was true when the bank note was for the gold or silver that was deposited in the bank. Now every bank note is just a paper about an amount of nothingness :)