Aside from the fact that at the moment we've all agreed to believe in the fiction that is money, it does have one "real" value - as legal tender, it must be accepted as payments for claimed debt. If you have no debt, money has as much value as people believe it does.
If someone claims you owe them money, they have to accept cash to pay the debt. They can take seashells or beanie babies, too, if they agree to it, but they can't refuse cash and still have a legal claim against you.
For transactions not yet made, cash is worth what the person you're trying to give it to thinks its worth.