Overdrawing the credit balance on your Oyster Card doesn’t mean you can travel around for free on the last day of your vacation. In this case, the amount owed will be deducted from the £5 deposit. You can settle this negative amount the next time you top up your card, or simply retain the card instead of returning it and forfeit your left-over credit. Unreturned cards are valid for up to 2 years since the date of last use. On expiry, they can be reactivated at a TfL center at no extra charge. Further information about the Oyster Card.