Bava Metzia, Chapter 7, Mishnah 6. Our Mishnah continues to discuss the right of a worker to eat from the produce he is working with. Its novel point: A person may waive this right and stipulate with the employer for a higher wage in exchange for refraining from eating, and even a husband or father may make such a stipulation on behalf of his wife or children. This point is not explicit in the Mishnah, and therefore it must be clarified: The one actually doing the work - the child, wife, or slave - must willingly agree to waive the right granted by the Torah, and only after they agree can the negotiations take place.
The opposite is derived from this as well: If someone lacks understanding and cannot waive his rights, such as a minor, his father may not stipulate with the employer on his behalf for a higher wage in exchange for less food, since the minor himself cannot waive the right the Torah gave him to begin with.
Cases where one may stipulate:
"Kotzetz adam al yedei atzmo" - the term for stipulating was already mentioned regarding fixed interest, and it means a prior condition. Here too: A person stipulates in advance with his employer and says to him, I agree not to eat from the grapes I am harvesting, and in return you will pay me forty shekels an hour instead of thirty.
"Al yedei beno uvito hagedolim" - a person may make such an arrangement also on behalf of his adult son or daughter working for that same employer, since they are adults, of bar or bas mitzvah age, and have the power to waive their rights. Again it must be emphasized: The father is not waiving his son's rights unilaterally, but rather the son agrees not to eat in exchange for an increase in pay, and the father acts as his spokesman in making the arrangement.
"Al yedei avdo veshifchato hagedolim" - the same applies to his slave and maidservant, as long as they are adults. Only if they agreed to waive their right to eat may their owner stipulate for a monetary increase in pay. For example, their employer might say to them: If you waive your right to eat from the produce you are working with, I will receive forty shekels an hour instead of thirty, and with this money I will pay for your air conditioning - and as long as they agree, this can be done.
"Al yedei ishto" - so too on behalf of his wife, and even if she is working for another employer, he may stipulate on her behalf, provided the wife agrees to waive her right to the food.
"Mipnei sheyesh bahen daas" - all of these can waive their rights because they have understanding.
Cases where one may not stipulate:
"Aval eino kotzetz al yedei beno uvito haktanim" - the father cannot waive these rights on behalf of his son or daughter who have not yet reached the age of mitzvos. Even if they themselves say they agree not to eat, they are not halachically competent to waive a right the Torah granted them, and therefore the father cannot make such an agreement.
"Velo al yedei avdo veshifchato haktanim" - the same applies to his slaves. One who rents out his ten-year-old slave may not tell the renter to pay a higher hourly wage in exchange for the slave's agreement not to eat, since a minor cannot make such an agreement and cannot waive his rights.
"Velo al yedei behemto" - and similarly with his animal: One who rents out his cow to his neighbor for threshing cannot stipulate with him that he muzzle the cow and in return pay a higher rental fee.
The reason in all these cases is "mipnei she'ein bahen daas" - the minor or the animal doing the work with the produce are incapable of waiving their rights. The Torah views it as unacceptable that one would work with food and be hungry while being forbidden from eating it, and therefore permitted it to them. Another person does not have the power to waive their right, and they themselves also cannot waive it due to being minors, and therefore the right to eat stands for them in any situation.
In summary: In this Mishnah we learned that the right of a worker to eat from the produce he is working with can be waived, provided the one doing the work has understanding and willingly agrees to it. A person stipulates on his own behalf, and even on behalf of his adult son and daughter, adult slave and maidservant, and wife - because they have understanding. But he does not stipulate on behalf of his minor children, minor slaves, and animal - because they lack understanding, and the right to eat that the Torah granted them remains intact.