You don't have to pay in dollars, but you must accept dollars to settle a debt. You and I can have a contract where I buy a cow from you today in exchange for

You don't have to pay in dollars, but you must accept dollars to settle a debt. You and I can have a contract where I buy a cow from you today in exchange for an iPod next week, that's a valid contract. But if I cannot produce the iPod next week, you must allow me to give you a quantity of legal tender currency (US coins or paper dollars) to settle the debt. If you require payment upon receipt, however, you can refuse to exchange the cow for cash.