That's mostly a problem of acceptance than value. I bet that if I pick a 100USD bill and go to the market and try to buy a bread, they won't sell me a bread that costs 1EUR. Why? Because only EUR is accepted in my country. A smart cashier might put 1EUR from his pocket and let me take the bread, keeping the 100USD for him. But that's another story... :-)

Just as the bank notes we have in our pockets have no value. They are just paper.