Maseches Maasros, Chapter 4, Mishnah 4 - part two. In this section we will discuss the power of Shabbos to establish the obligation to separate maaser.
"Tinokos she'tamnu te'einim l'Shabbos":
The word "tinok" in the language of the Mishnah does not mean what it means in modern Hebrew, where it refers to an infant; in the language of Chazal a "tinok" is a child who has not yet reached the age of mitzvos - a boy of about eight or ten. The Mishnah is therefore speaking about children who hid away figs and set them aside with the intention of saving them for Shabbos.
Setting them aside in itself does not prevent them from eating the figs before Shabbos, and nevertheless the Mishnah teaches: "v'shachechu l'asran - lo yochlu l'motza'ei Shabbos ad she'ye'asru" - once the figs were designated for Shabbos and no maaser was taken from them beforehand, they may not be eaten even after Shabbos until maaser is separated.
Let us clarify the point: before Shabbos, as long as Shabbos has not passed over them, there is no obligation to tithe them, and one may eat from them a casual snack without separating maaser. But since they were set aside for Shabbos, once Shabbos arrives they belong to Shabbos.
On Shabbos itself they cannot be eaten, because someone who did not tithe his food before Shabbos may not tithe it on Shabbos - by Torah law it resembles "tikun mana", repairing a vessel. It turns out that the figs cannot be eaten on Shabbos itself, and since they were designated for Shabbos, after Shabbos one is obligated to tithe them before eating them, even as a casual snack.
And this is the novelty of the Mishnah: even though we are dealing with tinokos - children who are not yet obligated in mitzvos - their intention carries enough weight to establish the obligation of maaser.
"Kalkalas Shabbos - Beis Shammai potrin u'Beis Hillel mechayvin":
Here we are dealing with a higher level. This is not a case of putting figs aside in a hidden spot, but of placing them in the kalkalas Shabbos - the Shabbos basket, something like the "Shabbos treats drawer" in today's terms. This designation is an official one: unlike the previous part of the Mishnah, a person does not normally reach into that basket and take from it before Shabbos, since it has been established that these items are for Shabbos.
The question, then, is this: once the fruits have been designated for Shabbos in a way that leaves no room to go back, may they be eaten as a casual snack without maaser if a person changes his mind before Shabbos?
Beis Shammai exempt: one may eat from them as a casual snack, and they do not become obligated in the separation of maaser.
Beis Hillel obligate: once they were officially designated for Shabbos and placed in the Shabbos basket, there is no going back, and whoever wants to eat them, even before Shabbos, must tithe them first. And this is the halachah.
The view of Rabbi Yehudah - a basket given as a gift:
Rabbi Yehudah extends the law even further: someone gathers figs and places them in a basket, and this is not a Shabbos basket but - as the Bartenura explains - a basket prepared as a gift. Since under ordinary circumstances a person does not help himself to the gift basket he intends to send, the fruit is now designated for giving as a gift.
And as we have learned in the past, giving something as a gift does not trigger the obligation to separate tithes. However, when a person sends a gift to his fellow, once it reaches the recipient's house, since it is already his, it becomes obligated in tithes according to the moment it arrived at his house.
The point here is that the designation is made in a formal way: the food leaves the possession of the giver and is designated in a particular way for the recipient, and it is already on its way to him. This is not a casual giving, and it may not be so far removed from a formal sale. For this reason, in Rabbi Yehudah's view, one may not take food out of that gift basket before tithing it, since it has been given a formal designation to some degree.
To summarize: the halachah does not follow Rabbi Yehudah, and in practice the laws divide as follows:
Designating food for the purpose of giving a gift - does not obligate the separation of tithes.
Designating food for Shabbos in a formal way - such as placing it in a Shabbos basket, or when it has already been explicitly marked as set aside for Shabbos - requires that it be tithed before one eats from it.
A more incidental designation for Shabbos - such as figs that the children put away - once Shabbos has passed, they must be tithed afterward before being eaten.
This, then, is the power of Shabbos to establish the obligation to separate tithes.