Many of us already know that we should always pay in the local currency rather than become a victim of dynamic currency conversion by paying a GBP amount determined by the merchant.
Last month my girlfriend paid a restaurant bill of EUR 26.50 with her Revolut card in Spain. AFTER she had authorised EUR 26.50 to be charged to her card and handed the card terminal back to the waiter, a message appeared on the terminal asking whether she instead wished to pay GBP 23.61 (which was 3.3% more). As the waiter was holding the card terminal, she clearly told the waiter “no” twice and that she wished to pay in EUR, given that she knew that this dynamic currency conversion is a scam used by many merchants, particularly in Spain, which serves only to generate additional revenue for the merchant. The waiter ignored her instruction and disingenuously pressed “yes”, causing her card to be charged GBP 23.61 without her authorisation. Although the amount was small, the principle of Spanish merchants scamming non-Eurozone tourists by 3.3% is not one that she wished to support. Therefore she insisted that the merchant refund the unauthorised transaction. The restaurant manager eventually refunded GBP 23.61 to her Revolut card (which took a couple of days to appear on her Revolut account). She paid the EUR 26.50 again, this time correctly.
It is particularly worrying that Spanish card terminals allow the merchant to change the currency and amount AFTER authorisation; the currency amount authorised by the cardholder should always be final. Be warned!