Ok, the specific law that relates to what is "lawful money" in the US, i.e. what is money in the eyes of the law, is specified in the Federal Reserve Act 1913. There is a body of Supreme Court rulings and precedences surrounding the 1913 Act, that you can dig out for yourself. Interestingly, the federal reserve private bank debt notes have been brought into being under the body of contract law not civil law, so as to not fall foul of the constitution whilst still retaining FR monopoly on credit issuance, but the 1913 Act specifies that FRNs can be "redeemed for lawful money".
In fact you do not even have to go look up the Federal reserve Act 1913 (but it could be instructive for your learning), you simply need to take a greenback out of your wallet and read the fine print on it!
"This note is legal tender for all debts public and private, and is
redeemable in lawful money at the United States treasury, or at any Federal Reserve bank."
Note that just by using FRNs contract law debt notes for commerce, but not redeeming for "lawful money", you are agreeing implicitly to enter into contract with the Federal Reserve and then have to play by their rules.
http://savingtosuitorsclub.net/forumdisplay.php?16-Private-Credit-vs-Lawful-MoneyIt is quite specific what is "lawful money" ... trying to claim bitcoin as "money" in court could land you in prison as a counterfeiter.
Now, do you want to try to tell the judge what is and isn't "lawful money" ?