I just had a situation in Hong Kong at the hostel I stayed at (The Mahjong) that reminded me a lot of the horror stories in this thread it had me very nervous:

1. They didn't take American Express
2. With my MasterCard, they inserted it and the terminal said "rate lookup" (oh no). They assured me I would get to choose.
3. I circled HKD on the receipt.
4. My copy of the receipt had both amounts and the statement (incorrect) that I had elected not to use MasterCard currency conversion.
5. I waited patiently for several days. The authorisation was in GBP (just over £189).
6. The final amount posted as $1700 (just over £178).

I saved just under £11 by opting out of DCC, and they did, in fact, honour the currency choice on the receipt despite showing all the hallmarks of non-compliant systems in this thread. Therefore clearly there *is* a way these merchants can honour your selection if they want to.
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