If you go to a ticket window, the member of staff (if safe to do so)
is meant to assist you in using a Passenger Operated Machine. If this
is broken they can either visit a newsagents or can travel and pay at
their destination, but what they CANT do is be sold a ticket from,
the, errmm, Ticket Office. The seller then leaves themselves open to
disciplinary action.

So what do they expect you to do if you have a Customer Charter Refund
voucher? It seems strange that they can only make refunds by paper voucher
and not by crediting an Oyster card directly.