Just to wrap this up ... The property was correct. My credit card statement was posted today and the amount in euros and the exchange rate are shown for the transaction. The amount was 795€. The exchange rate was higher than I expected leading to my confusion when all I could see was the amount in $. It's still a mystery to me why the property would only charge 795€ when the rate I agreed to, 820€, was 100% prepaid and non-refundable.