I've had this happen a number of times. Last time in London at a restaurant. They had already converted it to dollars for me to sign. I think it was about 4% extra.

I use a Cap One card for international charges, just for this reason. That doesn't stop merchants from getting even more. Worst one was Bumrungrad Hospital in Bangkok. Either pay the posted price in Baht in cash, or pay 3-4% more when they converted Baht to US Dollars.