Contact the issuer first
Report the problem to the card issuer's customer service (SAC) and keep the protocol number.
Contest in time
To dispute a card charge under the CDC, notify the card administrator at least 10 days before the bill's due date; you can then deduct the disputed amount and pay the rest.
Go to the ombudsman
If customer service doesn't solve it, escalate to the institution's ouvidoria.
Escalate outside the bank
Complain to the Banco Central (the institution must answer within 10 business days), your state Procon or consumidor.gov.br.
For scams, file a police report
The Banco Central recommends a Boletim de Ocorrência and contacting both the issuer and the card machine company.