I think it is in US CODE Title 31........ plus the coinage acts, most recent of 1965.
Here’s what snopes says on the matter, so take it will a grain of salt.
http://www.snopes.com/business/money/pennies.asp
snip
Up until the late 19th century, pennies and nickels weren’t legal tender at all. The Coinage Acts of 1873 and 1879 made them legal tender for debts up to 25 cents only, while the other fractional coins (dimes, quarters, and half dollars) were legal tender for amounts up to $10. This remained the law until the Coinage Act of 1965 specified that all U.S. coins are legal tender in any amount. However, even in cases where legal tender has been agreed to as a form of payment, private businesses are still free to specify which forms of legal tender they will accept. If a restaurant doesn’t want to take any currency larger than $20 bills, or they don’t want to take pennies at all, or they want to be paid in nothing but dimes, they’re entitled to do so (but, as mentioned earlier, they should specify their payment policies before entering into transactions with buyers). Businesses are free to accept or reject pennies as they see fit; no law specifies that pennies cease to be considered legal tender when proffered in quantities over a particular amount.
end snip
The difference is that legal tender laws don’t mean what people think they mean. This guy owes a debt to Chase - so can get away with it. But a business can set the terms of payment beforehand, specifying whether they accept cash, or checks, or credit cards, whatever. This is a source of confusion for folks.
ML/NJ
* The original Federal Reserve Notes were redeemable in GOLD, but WERE NOT legal tender. (They stayed this way until FDR outlawed possession of gold and BTW made FRNs legal tender.)