Ah, so essentially require the inclusion of a pre-signed, server-generated token identifying the user in the ticket’s QR code and perform the actual charge when the ticket is used.

That’s much better, though it’s subject to some edge cases similar to the London Underground, caused by the potential for using invalid payment methods. Not bad though; probably low-enough potential for abuse to ignore.

Off the top of my head, if the backend holds either the payment details or an alias to them from a payment service provider, you could also implement a solution where the turnstile/conductor is connected to a network and the payment gets done on the backend once a ticket generated offline gets scanned/used