Paper dollar is illegal.

Moreover, the "dollar" was not an arbitrary term at the time the Constitution was drafted. In the late 18th century, everyone knew what the "dollar" referred to: the silver Spanish milled dollar, which was in widespread use in the United States. The Constitution twice refers to the dollar — in Article I, Section 9, Clause 1 (a clause that everyone understood to involve a tax on the import of slaves), and in the Seventh Amendment (which protected the right to a jury trial in civil cases involving at least twenty dollars).

If the dollar had been something that Congress could manipulate at will, or if "dollar" had been merely a generic term to refer to whatever Congress should arbitrarily choose to recognize as currency, the South would never have accepted that clause — or the Constitution itself. Congress might have manipulated the dollar so as to make the tax on slave imports prohibitively expensive. It could also have effectively abolished trial by jury in civil cases by making twenty "dollars" an astronomically high amount of money.

mises.org/library/great-gold-robbery-1933