I agree with your not accepting these conversions, and I too am thinking of how best to fight back.

Objecting through your credit card's protection may work, though once you've accepted the charge in dollars at the point of sale, on what basis can you then object later? Moreover, your credit card company isn't causing the problem, and they aren't profiting from the scam.

My point is that I am thinking of refusing to sign for any purchase run through in dollars without my approval, demanding that the merchant re-run the transaction in local currency. It may take some time, and it may clog up their system, but it may also contribute to their asking before the fact at some point down the road.

Does anyone know for sure whether this is optional at the merchant's terminal (for those merchants that have this system). Put another way, if a merchant presents an overseas transaction in dollars, can the merchant instead run a US card through in local currency?