Yep, it worked. Thank you for the receipt.

A carbon copy receipt works heavily in your favor since the merchant would have the original clearly denoting your preference for HKD. It's a rock solid case for a Reason Code 76 chargeback. The conversion rate of 623 HKD is $80.38 according to Visa. It is indeed a markup of 4.2% above this rate at $83.76, meaning you were overcharged by $3.38 if the merchant hits you with DCC. I'd file a dispute, but it's unclear how your US issuer will proceed. For $3.38 they might issue a courtesy credit rather than going through with a chargeback, which is what we want.