Derivations from Deuteronomy 23:25-26
Translation Reprinted From Chabad.org

§1 When workers are performing activities with produce that grows from the earth,' but the work required for it has not been completed, and their actions bring the work to its completion, the employer is commanded to allow them to eat from the produce with which they are working. This applies whether they are working with produce that has been harvested or produce that is still attached to the ground.  This is derived from Deuteronomy 23:25, which states: "When you enter the vineyard of your colleague, you may eat grapes as you desire," and ibid: 26, which states: "When you enter the standing grain belonging to your colleague, you may break off stalks by hand." According to the Oral Tradition, we learned that these verses are speaking solely about a paid worker. For if the owner of the produce did not hire him, what right does the person have to enter his colleague's vineyard or standing grain without his permission? Instead, the interpretation of the verse is that when you enter the domain of your employer for work, you may eat.

§2 What are the differences in the application of this mitzvah between a person who performs work with produce that has been reaped and one who works with produce that is still attached to the ground? A person who performs work with produce that has been reaped may partake of the produce as long as the work necessary for it has not been completed. Once the work necessary for it has been completed, he may not eat. By contrast, a person who performs work with produce that is still attached to the ground - e.g., a harvester of grapes or a reaper of grain - may not partake of the produce until he has completed his work.

For example, a person harvests grapes and puts them into a large basket. When the basket is filled, it is taken away and emptied in another place. According to Scriptural Law, the worker may eat only when the basket has been filled. Nevertheless, in order to prevent the owner from suffering a loss, the Sages ruled that the workers may eat while they are walking from one row to another and while they are returning from the vat, so that they will not neglect their work to sit down and eat. Instead, they were granted permission to eat while they are performing their work, so that they will not neglect it.

§3 When a person neglects his work and eats or eats when he has not completed his work, he transgresses a negative commandment, as Deuteronomy 23:26 states:

"You shall not lift a sickle against your colleague's standing grain."

According to the Oral Tradition, it is explained that as long as the worker is involved in reaping, he should not lift a sickle in order to partake of the produce himself. Similar laws apply in all analogous situations.

Similarly, a worker who carries home produce with which he had worked or who takes more than he can eat himself and gives to others transgresses a negative commandment, as ibid.:28 states:"You may not place in your containers."
The violation of these two prohibitions is not punishable by lashes, because a person who ate when one should not have or took produce home is liable to make financial restitution.

§4 A person who milks an animal, one who makes butter, and one who makes cheese may not partake of that food, for it is not a product of the earth.

When a person hoes around onion heads and garlic heads, even though he removes small ones from the larger ones, or the like, he may not partake of them, because this activity does not constitute the completion of the task.

Needless to say, watchmen over gardens, orchards and fields where any crops are grown - e.g., cucumber gardens and gourd gardens - may not partake of the produce growing there at all.

§5 A person who separates dates and figs that have already been harvested and are stuck together] may not partake of them, for the work that obligates the performance of the mitzvah of tithing has been completed.

A person who works with wheat and the like after they have been tithed - e.g., a person was hired to remove pebbles from grain, to sift the kernels or to grind them - may partake of them, for the work that obligates the performance of the mitzvah of challah has not been completed. When, however, a person kneads dough, bastes loaves or bakes, he may not partake of the food, because the work that obligates the performance of the mitzvah of challah has become completed. And a worker may not partake of produce except when the work that obligates the performance of the mitzvah of tithing or challah has not been completed.

§6 If the cakes of figs belonging to a person become broken up, his barrels of wine become open, or his gourds become cut, and he hires workers to tend to the produce, they may not partake of it, for the work necessary for them has been completed and they have become obligated to be tithed. Indeed, they are Tevel.

If, however, the owner did not notify the workers, he must tithe the produce and allow them to partake of it.

Workers may not partake of the crops in a field that was consecrated to the Temple treasury. This is derived from Deuteronomy 23:25, which speaks of "your colleague's vineyard."

§7 When a person hires workers to work with produce that is Neta Reva'i, they may not partake of it. If he did not inform them that it was Neta Reva'i, he must redeem it, and allow them to partake of it.

§8 Workers who reap, thresh, winnow, separate unwanted matter from food, harvest olives or grapes, tread grapes, or perform any other tasks of this nature are granted the right to partake of the produce with which they working by Scriptural Law.

§9 Watchmen for vats, grain heaps and any produce that has been separated from the ground, for which the work that obligates tithing has not been completed may partake of the produce because of local convention. They are not granted this privilege according to Scriptural Law, because a watchman is not considered to be one who performs an action.

If, however, a person works with his limbs whether with his hands, his feet or even with his shoulders, he is entitled to partake of produce according to the Torah.

§10 A worker who is working with figs may not partake of grapes. One who is working with grapes may not partake of figs. These laws are derived from Deuteronomy 23:25, which states:

"When you enter the vineyard of your colleague, you may eat grapes."

When a person is working with one vine, he may not eat from another vine. Nor may he partake of grapes together with other food; he should not partake of them together with bread or salt. If, however, the worker set a limit concerning the quantity that he may eat, he may eat the produce with salt, with bread or with any other food that he desires.

It is forbidden for a worker to suck the juice from grapes, for the verse states: "And you shall eat grapes." Neither the worker's sons nor his wife may roast the kernels of grain in a fire for him. This is implied by the above verse, which states: "You may eat grapes as you desire." The implication is that you must desire the grapes as they are. Similar laws apply in all analogous situations.

§11 It is forbidden for a worker to eat an inordinate amount of the produce with which he is working. This is implied by the above verse, which states: "You may eat... as you desire, to your satisfaction." It is permitted, however, for him to delay eating until he reaches the place of higher quality grapes and eat there.

A worker may eat even a dinar's worth of cucumbers or dates even though he was hired to work only for a silver me'ah. Nevertheless, we teach a person not to be a glutton, so that he will not close the doors in his own face. If a person is guarding four or five grain heaps, he should not eat his fill from only one of them. Instead, he should eat an equal amount from each one.

§12 Workers who have not walked both lengthwise and laterally in a vat may eat grapes but may not drink wine, for at that time they are still working solely with grapes. When they have treaded in the vat and walked both lengthwise and laterally, they may eat grapes and drink the grape juice, for they are working with both the grapes and the wine.

§13 When a worker says: "Give my wife and my children what I would eat," or "I will give a small amount of what I have taken to eat to my wife and my children," he is not given this prerogative. For the Torah has granted this right only to a worker himself. Even when a Nazarite who is working with grapes says, "Give some to my wife and children," his words are of no consequence.

§14 When a worker - and his wife, his children and his slaves - were all employed to work with produce, and the worker stipulated that they - neither he nor the members of his household - should not partake of the produce, they may not partake of it.

When does the above apply? When they are past majority, because they are intellectually mature, responsible for their decisions, and willingly gave up the right the Torah granted them. If, however, the children are minors, their father cannot pledge that they will not eat, for they are not eating from his property or from what the employer grants them, but rather from what they were granted by God.