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) wit

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.