What is the ruling on purchasing a device with a credit card based on the the assumption that there are no interest charges, and then it becomes clear later that it was an interest-based transaction, and how can one exit from this sin?

It is not permissible to deal with usurious banks except out of a recognized necessity, because dealing with them involves aiding and strengthening them in their falsehood. The resolution of the Islamic Fiqh Academy states that it is forbidden for a Muslim to deal with usurious banks when an Islamic alternative is available. If the bank declares the existence of usurious interest on the loan, then one must strive to get rid of the transaction by accelerating repayment, if possible, to cancel or reduce the interest, and not to deal with the card again. It is hoped that there will be no sin for what has passed due to ignorance of the transaction.