[rbanffy]

With any significantly complex architecture, there are many ways of being off-line. You may be unable to check a balance, but able to validate a PIN, or unable to make a transfer, yet perfectly capable of doing every other transaction. In that situation, if you can validate the PIN without being able to check the balance and you know this type of card is issued to clients with a certain overdraft limit, it's safe to clear the transaction and just tell the backend it happened when all remote functions are back online.