The pertinent portion of law that applies to the question is the Coinage Act of 1965, specifically Section 31 U.S.C. 5103, entitled "Legal Tender", which states: "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 statute means that all United States money as identified above "are a valid and legal offer of payment for debts when tendered to a creditor."
https://www.treasury.gov/resource-center/faqs/Currency/Pages/legal-tender.aspx
Read the above carefully, the United State money as identified above, primarily the part about “United States coins and currency (including Federal reserve notes and circulating notes of Federal reserve banks and national banks)”
The listed circulating note and coins are only "a valid and legal offer of payment. There is an OFFER of payment, once accepted AS payment, they become "lawful" via agreement (acceptance is contract). However, they do not attain "lawful" in the Constitutional definition, but Congress cannot impair obligations of contracts (Article 1, Section 10, Clause 1 of US Constitution).