I took the view that the charge in dispute was the small percentage added through DCC. If the retailer won't correct it at the time, I inform the card company so they are made aware of who isn't correctly following their procedures. How they resolve it is up to the card company.

I do the same when a restaurant unilaterally decides to add its own tip to a card payment.

In neither case am I disputing the whole transaction, only the overcharge.