I am a Hypnotherapist - and I will not bill my client until after the session.
If they feel there has been no change - I will not bill them at all - even though I may know they did get some change but are unaware consciously, or it was their own resistance that blocked change - it doesn't matter to me - I want them to pay only when they feel they got true worth from the process.
Now I have been in practice for over 20 years - and it's a rare event to not bill. But I am happy to do so - if the  client didn't get a result