It is a scam if the merchant does not follow the rules which clearly state the transaction must be carried out in local currency unless the customer agrees to be ripped off by dcc. The terminals (perhaps not in China as somebody points out but throughout Europe) are programmed at the start of the transaction, if the merchant's processor pushes dcc, to state what the amount would be in both local currency and the currency of the credit card and then the cashier must push one of two button on the terminal...yes the customer wants to be ripped off or no the customer is smart and unerstand it is a rip off. Many merchants just instruct their cashiers to assume the customer wants to be ripped off oops wants to pay in his or her currency and that's when the fun can begin.

The sales slip will clearly show both amounts and you will be pressured to sign a statement on the slip stating you were offered a choice and agree to the conversion and this choice is final. Do not sign. Politely ask the clerk to void (not credit) the transaction and do it properly. He may throw out one of the lies such as the amount in your currency is just for your guidance but the transaction is going through in local currency, it's a good rate, it avoids foreign currency fees, he has no control over it, it is required by law, transaction cannot be voided. If he throws out one of these lies, ask to see the manager. The manager may repeat some of the lies or if you're not in Ireland (where this scam began), Scotland, Wales or England you may get the no speak English lie.

What you do is noted above. Do not offer to pay in cash, this plays in to what the merchant is trying to do. Rather circle the local currency amount, cross out your currency amount and write choice not offered. If you have a camera phone, take a picture of the receipt. Somtimes this is the straw that breaks the camel's back. But if it doesn't, when you get home, dispute the charge. You have to win.

Incidentally, Amex does not allow dcc on any of its cards.