This is such a good thread I thought I would add my 2 cents. They tried this scam on me at a Cologne Hotel. the new "wrinkle" was that I had booked the hotel through Booking.com and had agreed that they could charge my credit card card on booking. Card is capital 1 so I pay no Foreign exchange fees. When they presented the receipt in the morning it had DCC with the usual rip off rate. I got all the standard lies . "The bank makes us do it", "it's not us it's the bank" "It's the standard exchange rate" etc etc etc. I do have some advantages. I speak German and I've taught law and technology in Germany and the USA. My German lawyer lives only a few miles away. After listening to Hotelier crap for 10 minutes I suggested calling the criminal police and making it a police matter. They quickly refunded the charge and I paid cash. I have had 10 emails exchanged with Booking.com asking why they do not prohibit their contract partners from making these totally unauthorized transactions. They have yet to give a satisfactory answer.
I urge everyone to fight this scam/fraud with every tool at your disposal. You will help every traveler