This conversion can only give you a reference exchange rate, because when you swipe your card, MasterCard is not converted into US dollars according to the exchange rate published by the Bank of Hong Kong on the day you swipe your card. They have their own exchange rate, but don't worry, the exchange rate they exchange is more cost-effective than that of Hong Kong banks, but they will also charge exchange fees when deducting the accounts. The specific exchange rate is the day when your credit card is debited (excluding dollars), because sometimes it is not debited from your credit card on the same day. Because credit card repayment has an interest-free repayment period, you can observe the appreciation of RMB on that day during the interest-free repayment period, then convert the US dollars in your account into RMB and RMB according to the exchange rate announced by the bank where your credit card is located, and then call the credit card center to request that RMB be converted into US dollars for repayment. Or take the bill and go to the counter immediately to change it into dollars and return it directly to dollars. In short, you can't control the exchange of Hong Kong dollars into US dollars, but you can control the exchange of US dollars into RMB. Note that when returning US dollars, converting US dollars into RMB is the price at which RMB buys US dollars. The exact amount you want to repay is shown on the bill.