(MONEY Magazine) – If you used your credit card overseas between 1996 and 2000, you could be getting a piece of a $500 million penalty against Visa and MasterCard.

THE STORY: When you make a purchase abroad, Visa and MasterCard tack on a 1% fee for foreign-currency transactions. But the card companies haven't properly disclosed that charge to you, according to a preliminary ruling from a California judge. So they may have to pay some of it back. (Several banks now mention the charge in cardholder agreements; only a few list it on the bill.)

THE CATCH: The judge's final decision--as well as the details of who gets the refund, how much money it will be and how it will be paid--is set to come in early March (after MONEY goes to press). But the total refund could be $500 million--a small sum, considering that the card companies pulled in an estimated $1 billion in foreign exchange fees over those four years from U.S. cardholders. And if the preliminary ruling holds, Visa and MasterCard plan to appeal. "We have strong legal arguments to have this judgment overturned," says a MasterCard spokeswoman.

THE UPSHOT: Don't expect the fees to go anywhere--they're legal. Even so, plastic still beats currency-exchange services and ATMs as the cheapest way to pay overseas. --MAYA JACKSON