The thing about national fiat currencies like your $100 bill is that millions of people are forced by the state to transact in that currency and none other, and it is backed by the promises and threats of the state. When the confidence in that backing evaporates (as happened with Assignats for example in post-revolutionary France, originally backed by property, later backed by thin air), bad things happen and what seemed valuable can overnight lose its value. All that is solid melts into air.