When Lisa Selby* used her debit card to pay for two slices of barbecued cheese from a beach vendor in Rio de Janeiro, she expected to pay 40 reais (£5.90) for the snack.

But shortly after the payment had gone through, she realised that she had been charged 4,000 reais (£590) after the vendor added two extra zeros to the card reader.

“He showed me the right number on the contactless terminal, but then turned it round to face him and, unbeknownst to me, added two zeros just as I was about to tap with my phone,” she said. “He then gestured that there was no paper for a receipt and walked away.”

Holidaymakers have been warned to beware of the crime in Rio. Last month a beach vendor was arrested after a British man was duped into paying £1,500 for a kebab, while an Argentinian tourist discovered a £3 corn on the cob had cost her £3,000.

What it looks like

Unscrupulous traders take advantage of tourists who are unfamiliar with the local currency and are less likely to notice a doctored sum.

A common ploy is to ask the customer to confirm the correct figure on the card reader, then to alter it surreptitiously before the card is presented.

Some scammers thrust their card reader at the customer’s card or phone to complete the payment before the customer has checked the total.

What to do

The scam exposes a gap in UK consumer protection. Customers who are tricked into sending money to criminals by bank transfer in what’s known as authorised push payment fraud are usually entitled to a refund from their bank. However, victims of face-to-face vending scams are unlikely to get their money back unless they can produce evidence that they were overcharged.

Selby discovered the fraud shortly after the payment and immediately alerted her bank, Monzo. An agent told her that since the transaction was still pending it would be returned to her. Monzo later admitted that she had been wrongly advised and said that authorised payments cannot be reversed.

Her attempts to claim the money via her card issuer’s chargeback scheme, which allows a dispute to be raised with a trader who is in breach of contract, failed because she did not have an invoice or receipt showing the agreed price.

Monzo refunded her as a goodwill gesture after contact from Guardian Money. It said that reports of beach vendor scams are increasing but that Selby’s experience is classed as a buyer/seller dispute rather than fraud because she had authorised the card payment.

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.”

The Financial Conduct Authority, which regulates UK banks, confirmed that pending transactions cannot usually be reversed, and that chargebacks are a voluntary service offered by card issuers, which are free to decide their own criteria and processes.

It said victims may be able to submit a claim to their bank for an unauthorised transaction since they had not consented to the amount that was debited and that they can complain to the Financial Ombudsman Service if they are unhappy with the response.

To avoid being caught out, holidaymakers are advised to pay mobile vendors in cash, or to insist on holding the card reader to insert or swipe their card themselves to prevent a last-minute alteration to the price.

* Name has been changed