No federal mandate...state & local laws may vary:
https://www.federalreserve.gov/faqs/currency_12772.htm
https://www.patriotsoftware.com/blog/accounting/can-business-refuse-cash-is-it-legal/
A sincere “thank you”; I learned something new today. And any day I learn something I didn’t previously know, is a good day.
As long as the transaction is legally “a dollar”, whether it’s cash, check, coin, or I guess a virtual transaction - as long as both parties agree - seems it’s legal.
I understand, not sure I agree with the wisdom, but I understand.