Did you follow the suggested procedure at the duty free shop i.e. asking the transaction be voided and done properly in HK dollars and if they refused, cross out the statement on the slip given to you to sign saying you were offered the opportunity to pay in HK$, circle the amount in HK$ and write local option not offered. Then you dispute the charge as not being authorized and demand to see the signed merchant copy which they will be unable to produce. I don't see where Chase has any choice but to charge back the sale to the visa (or mastercard) system under visa or mc rules.