There is a difference between the correction of an error and a simple refund. In the former, the merchant vendor ought to make the consumer's loss right, e.g., the consumer was double-charged and should not suffer a currency exchange loss because the merchane vendor made an error.

Here, OP simply purchased a product, chose to "return" it as he is permitted to and received a refund. He received that refund and chose to do all of this with a US$ CC payable in EUR. As a currency arbitrageur, he lost on this deal.

Either purchase in the card's "home" currency or only purchase in currencies which your crystal ball tells you will increase or at least remain stable.

You will find precious few threads on FT which start with, "I purchased in XXX and it went up, what do I do with the extra US$???"