According to
www.moneyfactory.gov...
Legal Tender: A Definition
31 USC § 5103. Legal Tender
United States coins and currency (including Federal reserve notes and circulating notes of Federal reserve banks and national banks) are legal tender
for all debts, public charges, taxes, and dues. Foreign gold or silver coins are not legal tender for debts.
However, there is no Federal statute which mandates that private businesses must accept cash as a form of payment. Private businesses are free to
develop their own policies on whether or not to accept cash unless there is a State law which says otherwise.
So if foreign gold or silver coins are not legal tender for debts incurred on American soil, then how now- magically- can foreign bills be used as
legal tender?

This restaurant will likely run into some trouble with this, as many are bound to see this as illegal.
It would seem to me that everyone in the US can't just decide to accept foreign currency on their own as payment for local goods and services sold on
US soil. Or can they?
On the other hand, couldn't it also be argued that the US marketplace has been accepting Canadian coins, at least, for quite some time? Hand a
cashier a canadian nickel, and it is always taken. At least in my own experience to this point in the US. Maybe a quarter here and there. But that's
about as far as I've seen it go. These guys are flat out saying they are going to take a foreign currency in bills as a payment for pizza. To be or
not to be, that is the question.