It don't matter if you charge or work free the liability issue is still there.
Especially once you get off of your own property.
I have questioned my insurance agent at length on exactly this subject, and with my policy it makes a big difference.
Essentially, if I am doing a favor for a neighbor or a friend, and not charging anything, my homeowners liability goes with me and the tractor. The second I charge, it becomes commercial and I need different, and more costly insurance.
Informal trading of services is OK. My neighbor has a smaller tractor with a standard FEL, I have a backhoe with a 4-n-1. If dig his fire pit with my backhoe and the next week he carries a FEL load of wet concrete up to my place, we are OK. If we start to write down hours and try to make them even we are on thinner ice.
What doesn't go with the tractor is theft & comprehensive insurance. If I leave it at his place overnight and it is stolen, I am out of luck. If I drive it back to my place and it is stolen I am covered.
Now where it gets really interesting, and I have no answers is we live on a private road -- ownership is shared equally among 4 properties. What happens if I leave it on the road right-of-way and it is stolen? Agent won't tell me, so I don't leave it out overnight.