Qitzur Shulchan Arukh – 185:6
Just as the homeowner is warned not to steal the wages of the poor nor to delay it, so to the poor person is warned not to desist from the work of the homeowner. He must work with all his ability. As Yaaqov our forefather, peace be upon him, said, “[And you know that with all my ability I served your father.” (Bereishis 31:6)
Therefore a worker is not allowed to do work at night and hire himself out during the day, for he is already weakened from the night. Similarly, he can’t do work with his animal at night and hire it out / lease it during the day. The worker is also not allowed to starve or torture oneself, for he weakens his abilities and isn’t able to do the homeowner’s work properly.
The law is similar for a teacher, see previously, 165:12
I see that while 165 deals with teaching Torah rather than fiscal law, and therefore wasn’t in my original plan, it does contain three se’ifim of interpersonal mitzvos. I will present them tomorrow, and then return to our regularly scheduled programming.