Speaking as a lawyer, you'll all be pleased to hear that the official answer is "yes and no."
Pull out a dollar bill from your pocket. Any denomination. It's got some small lettering that appears to answer the question: "This note is legal tender for all debts, public and private." That's not-so-legalese for, "This stuff must be accepted in satisfaction of a debt." But that means for a debt
already existing. I don't care if you're trying to protect your staff from contracting some ultra-rare, highly contagious disease that is transmitted exclusively through paper currency, ya can't legally refuse cash when the debt is already contracted.
As you surmised, the "already existing" is the catch. You can't legally refuse cash
so long as the buyer was not made aware of your terms. In other words, you can impose any conditions you want--but ONLY in advance. If they post a prominent sign saying "We accept payment only in shark's teeth," they are entitled to enforce that and CAN legally refuse payment in cash. On the other hand, if they don't properly notify (we'll skip over what "properly" means in this context) people of the rules, any poor slob who goes in and isn't made aware CAN force them to accept cash. Confused?
If you don't believe me, take a look at the
website of the Federal Reserve Board:
"Is U.S. currency legal tender for all debts?
According to the "Legal Tender Statute" (section 5103 of title 31 of the U.S. Code), "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." This means that all U.S. money, as identified above, when tendered to a creditor legally satisfies a debt to the extent of the amount (face value) tendered.
However, no federal law mandates that a person or an organization must accept currency or coins as payment for goods or services
not yet provided. For example, a bus line may prohibit payment of fares in pennies or dollar bills."
P.S. Where do I send the bill?
Gypsy Boy
"I am not a glutton--I am an explorer of food." (Erma Bombeck)