The prompt is built to sound like a favor. You are at a till in Lisbon or a cash machine in Prague, your card goes in, and the screen asks whether you would rather be charged in your own currency instead of the local one. Say yes and you get to see the total in the money you think in. Say yes and you also hand over a markup the receipt will never name.

This is dynamic currency conversion, DCC, and it is the terminal's version of the budget-airline upsell: a charge priced so the person paying it cannot see it. When you accept, the merchant's or the machine's bank sets the rate, not your card network, and sets it in its own favor. NerdWallet puts that markup at 1 to 3 percent, stacked on top of any foreign transaction fee your card already charges. WalletHub reckons it can run up to about 7 percent above the real rate.

The number that settles it comes from a field test, not a marketing page. The German consumer group Stiftung Warentest sent 20 investigators to 11 countries and paid both ways; the European consumer body BEUC published what they found. In every case where DCC was used, the price went up, by 2.6 to 12 percent, worst in the Czech Republic, Poland, and Hungary. A Norwegian bank that checked 1,500 withdrawals abroad found the traveler lost out in 99.7 percent of them. Not sometimes a little: almost always more, sometimes a lot.

The rule is one line: always pay in the local currency. Euros in Portugal, koruna in the Czech Republic, yen in Japan. Choose local and your card network, Visa or Mastercard, converts at its own wholesale rate, close to interbank and about as good as a traveler gets. Choose your home currency and you have paid for the privilege of a familiar number at a worse rate.

You are allowed to refuse, and the rules are on your side even where the screen pretends otherwise. Visa's own cash-machine rule, in force since 2019, says the machine must tell you DCC is optional and must not steer you into it by default. In the European Union, since April 2020, your bank has to show the conversion cost as a percentage over the European Central Bank's reference rate. The law assumes you will say no; the interface is built to make you say yes anyway, with the home-currency button bigger, greener, already highlighted. Read past it and pick local.

Three tactics carry the rest.

Carry a card with no foreign transaction fee, so the network's good rate is not clawed back by your own bank. That is a separate charge from the DCC markup, though it happens to land in the same 1 to 3 percent band; a fee-free travel card removes it, and DCC stacked on top is the worst case there is.

Use a bank's own cash machine, not the standalone boxes in stations and tourist streets. The independent operators, and Euronet is the one you will meet all over Europe, push DCC hardest and add a withdrawal fee besides. Euronet's own published markups run as high as 14.95 percent. Decline the conversion, then decline again if it asks twice.

Skip the airport currency kiosk entirely. The counters that advertise no commission make their money on the spread instead, and cash bought on the day runs often 5 to 10 percent off the mid-market rate, worst of all at the airport, where you are captive and have no other option. Draw local cash from a bank's own cash machine in town, or pay by card in the local currency and let the network handle the conversion.

None of this asks you to think in a foreign currency, the fear the prompt preys on; your statement converts it back soon enough, at a fair rate, for nothing. So when the screen offers you the comfort of your own money, read it for what it is, a fee with a friendly face, and press the other button. That is the whole trick.