The scenario that inspired this question:
Yesterday I took a train journey, it matters not where, and on the outward portion the guard did not come to sell me a ticket, so on the return portion I asked to buy a ticket that would cover both journeys and while I forget the exact words of the guard was more or less told not to worry about it and just get a single. Never mind whether what happened in the particular occasion counts as permission; the question is whether the guard on the second journey has the power to say I could make a journey I already had effectively for free so that if secretly somehow I had been tailed by Scotrail and they now sent me an invoice for the first journey, which I believe they are entitled to do in theory, even though I did not break any rules, the journey had subsequently been authorised.