@Cheesesteak. You’re about to relitigate the exact scenario / thread @md-2000 mentions just above. See starting here
then proceeding intermittently for a while then really picking up speed around post 58 & subsequent.
It appears the legal answer is in the absence of local law requiring them to accept US cash, you’re either paying with Venmo, leaving the merch behind, or shoplifting. Take your choice of actions and corresponding consequences.
Bottom line is they are no more required by law to accept US cash than they are to accept e.g. Japanese cash.