[aptwebapps]

By offline do we just mean the passenger's device? If the POS is online, then what might work would be to hash the ticket's details (user id, journey, time) with a per customer secret. Embed that in the QR code and it's basically a claim that, yes, this passenger would like to be billed for this ticket. The POS would then check the passenger's account balance or for a previously authorized credit card or whatever.