When you take a look at your one dollar bill, it doesn't actually say "dollar bill" it actually says "one dollar" and further at the top of the bill, it literally states: "Federal Reserve Note", so the one dollar bill should really be known as the one dollar note. Additionally, on the front or obverse of the dollar it states: "This note is legal tender for all debts, public and private." This wording is the exact same wording that is placed on all the denominations of Federal Reserve Notes, so obviously, quite clearly, all businesses and all transactions in America, must accept physical dollar notes as an acceptable form of payment for debts so incurred, but in actuality, that isn't really true.
For instance, on airplanes nowadays, most airlines do not accept cash whatsoever, and therefore the payment for a drink or other items must be made by a debit or credit card. Then there are a small percentage of restaurants and other businesses that do not as a matter of their business policy, accept any cash, whatsoever, and this they make clear to their patrons before business is transacted. In addition, there are establishments, such as gas stations, that accept dollar notes, but only up to the denomination of $20, and will not accept any notes, such as $50 or higher. All of these transactions which limit or don't permit the usage of dollar notes, would appear to be against the law, especially since the dollar denominated note makes the statement that it is "legal tender for all debts, public and private."
The thing is that case law has already been established that private businesses, people, and organizations are free to set the stipulations of what they will or won't accept as a matter of their policy as an acceptable form of payment for those services, in which, consumers of such, must accept these terms or else conduct their business somewhere else. This would appear to be discriminatory especially against those that do not have ready access to debit cards, credit cards, checks, or other instruments other than cash, but nevertheless, as long as such a policy does not violate the monetary laws within that State, there are no Federal laws that compels such a business or individual to actually accept cash as a form of payment.
This then sets up a very interesting situation in which cash may not really be king, and could, under certain circumstances, be of worth that is far less than face value, for when businesses and people already have the option to not accept cash as a form of legitimate payment, it doesn't take any real stretch of imagination to recognize that those that only have cash, or preferred to just utilize cash, are placed into the rather unfavorable situation in which they must trade or deal with their cash in such a manner that in order to conduct business their cash is subsequently discounted in value to obtain a substitute for cash that is readily accepted by businesses. This means that the trading of cash for other forms of payment would be somewhat akin to money laundering, even though cash is actual legal tender, nevertheless, when it is no longer remains the preferred method of payment, its value must drop.