Third-party bus fare refunds in London

[GuyBarry]

A question that has mystified me for some time. It occurred to me that someone on this forum might know the answer.

I've attended courses where fares are refunded in cash on the spot on production of a travel ticket. I live outside London where it's still possible to get a bus ticket if you ask the driver for one.

How does it work in London, where there are no bus tickets? I know it's possible to get a statement from TfL but that wouldn't work in situations where people have no spare cash and need the money back immediately.