And this applies not just to transactions with the government, but to any (U.S.) transactions whatsoever. You can see evidence of this on every dollar bill: "This note is legal tender for all debts, public and private [emphasis added]". It's illegal to refuse dollars, even for private debts.
Threatening people with imprisonment to force them to accept dollars should seem fishy, though, and it's a hint that there's something rotten about fiat currency. Unlike commodity money, whose supply is limited by physical scarcity, fiat money can be expanded without limit, and the result is both sad and predictable. Consider: (1) monetary expansion benefits debtors, who can repay debt in inflated dollars; (2) U.S. monetary policy is set by the Federal Reserve, whose officials are appointed by the U.S. Federal Government; (3) the U.S. Federal Government is the world's largest debtor. It doesn't take a rocket scientist to figure out what is going to happen in this scenario.
While no one can legally refuse dollars to settle debt, it's perfectly legal to refuse any other dollar transaction. If I trade my chickens for fish and fish only there is nothing illegal about this.
Threatening people with imprisonment to force them to accept dollars should seem fishy, though, and it's a hint that there's something rotten about fiat currency. Unlike commodity money, whose supply is limited by physical scarcity, fiat money can be expanded without limit, and the result is both sad and predictable. Consider: (1) monetary expansion benefits debtors, who can repay debt in inflated dollars; (2) U.S. monetary policy is set by the Federal Reserve, whose officials are appointed by the U.S. Federal Government; (3) the U.S. Federal Government is the world's largest debtor. It doesn't take a rocket scientist to figure out what is going to happen in this scenario.