Not even the U.S. Government's own Internal Revenue Service is accepting the proposition that Federal Reserve notes are legal tender for the payment of all debts. If you want to pay your federal tax bill in cash, you can't pay more than $1,000 a day, and the government charges taxpayers an extra $3.99 for each cash payment. The IRS recently issued a reminder on this front. At a gas station, sometimes you get a discount for paying with cash. Leave it to the IRS to find a way to charge extra to taxpayers for doing something that earns a discount in the private sector.
The War on Cash, IRS Style