Okay, so whilst I'm not currently experiencing any such scenario, I'm not sure how I would proceed if it actually happened.
So say you had a friend who is a builder, and you want a fireplace installing or something. Providing it takes a day at the most, how do you go about paying them? Do you pay him more because he's a buddy? Pay less or nothing because he's just doing you a favour? Or meet him halfway and just pay the normal amount? What if he was a waiter serving you instead of a builder? Would you tip him more or less?
As I said I'm not sure how I'd go about this, but I imagine the more likely option is for him to do it cheap or free, and then I'll owe him in any way I can.