In that case, a $1 FRN is always a $1 FRN.
So, at least one part, one variable is fixed
If it doesn't keep prices fixed, what is it good for?
Many things go into price. Supply, demand, quality, quantity. But the value of the medium of exchange should remain the same over space and time. Something that the Federal Reserve’s notes haven’t done, despite their supposed mission, or half of it, anyways.
It’s hard to measure, price, something when the measuring units are changed all the time.