Lisa Selby expected to pay 40 reais (£5.90) for two slices of barbecued cheese from a beach vendor in Rio de Janeiro. The vendor showed her the correct price on the contactless terminal, then turned it to face him and added two zeros to the amount before she tapped her phone. She was charged 4,000 reais (£590) instead.

Beach vendor scams are escalating in Rio. Last month a British man was duped into paying £1,500 for a kebab, and an Argentinian tourist discovered she paid £3,000 for corn on the cob priced at £3.

The typical method involves asking the customer to confirm the correct figure on the card reader, then altering it before the card is presented. Some scammers thrust the card reader at the customer's card or phone to complete the payment before the total is checked.

Selby's bank, Monzo, initially told her the pending transaction would be returned. The bank later admitted she had been wrongly advised and said authorised payments cannot be reversed. Monzo refunded her as a goodwill gesture after contact from Guardian Money. A spokesperson said, "We understand this was a very frustrating experience for the customer. While our reasons for rejecting the claim were correct, we recognise that our communication fell short of our usual high standards."

Selby's attempt to claim the money through her card issuer's chargeback scheme failed because she lacked an invoice or receipt showing the agreed price. The Financial Conduct Authority confirmed that chargebacks are a voluntary service card issuers offer with their own criteria and processes.

Victims may submit claims to their bank for unauthorised transactions since they did not consent to the debited amount. Complaints can be lodged with the Financial Ombudsman Service if unhappy with a bank's response.

To protect themselves, holidaymakers should pay mobile vendors in cash or insist on holding the card reader themselves to prevent last-minute price alterations.