C A D
i agree with your general assesment of this topic but for different reasons
i dont know about the Halacha, but when someone volunteers to do a job he has a certain responsibility to make sure he does it properly. after all the job is important to the one that asked for it to be done, and had he known the volunteer would do a subpar job he might have arranged for a different person to do the work.
so, at least in a moral sense, you do “owe” something to the “employer”