Maseches Ma'asros, chapter two, mishnah 6. This mishnah is a direct continuation of the previous one. In the previous mishnah we learned that a person who gives money to his fellow in order to buy figs, saying to him, "Give me five figs" - the seller is the one who selects the figs and hands them to the buyer, and the buyer receives them one at a time and eats them without tithing, at least according to Rabbi Yehudah, and so the halachah is decided.
"Ha'omer la'chaveiro: heilach issar zeh v'eser te'einim she'avor li - boreir v'ocheil" - one who says to his fellow, "Here is this issar, and ten figs that I will select for myself" - he selects and eats.
The buyer approaches the owner of the fig trees and says to him: here is an issar for you, and I will select ten figs for myself from the tree. The underlying principle here is identical to that of the previous mishnah - it is not defined which particular figs are being purchased. First he hands over the money, and afterwards he picks them himself, and as he picks them they become his. Therefore, "boreir v'ocheil" - he selects and eats them one at a time, and he is not obligated to separate ma'asros.
Why does even Rabbi Meir not disagree here?
In the previous mishnah, where the seller is the one who selects, Rabbi Yehudah held that the seller's act of choosing shows that the fig has reached the end of its road, and this is a form of completion of the work. But in our mishnah, where the buyer is the one who picks the fig from the tree, who says that from the seller's perspective this is a completion of the work? Perhaps the seller had other plans for these fruits, to dry them or to store them. For this reason even Rabbi Meir agrees that the buyer's picking of the figs and eating them does not constitute a completion of the work, and therefore one at a time is permitted.
And it is understood that if he takes more than one at a time and has two in his hand - as in the previous mishnah - a collection is created, a gathering of purchased figs, and now they require tithing.
The mishnah illustrates this principle through four cases, in which the novelty grows progressively greater:
"Eser te'einim she'avor li - boreir v'ocheil" - ten figs that I will select for myself: he picks one fig at a time and eats it.
"Eshkol she'avor li - m'gargeir v'ocheil" - a cluster that I will select for myself: he picks off single grapes and eats. One who stands beside the vine and wishes to select a cluster of grapes for himself may eat without separating ma'aser, provided that he is "m'gargeir," that is, picking one berry at a time (a gargir is a small fruit, and the mishnah turned the noun into a verb). He picks a grape, eats it, and then picks another grape - and he never has in his hand a collection of picked grapes, and therefore the rabbinic obligation to tithe never takes effect.
"Rimon she'avor li - poreit v'ocheil" - a pomegranate that I will select for myself: he breaks off pieces and eats. Here the novelty is greater. The pomegranate fruit resembles a cluster of grapes: inside the peel are seeds, and each seed is a fruit in its own right, just as each grape is a fruit in its own right from the cluster. Therefore, one who wishes to eat from the pomegranate without tithing gives the money, selects a pomegranate for himself, and breaks off from it - while it is still attached to the tree - one seed at a time and eats it. But if he takes two seeds at once, this is a collection and they require tithing.
"Avatiach she'avor li - sofeit v'ocheil" - a melon that I will select for myself: he slices off pieces and eats. This is the greatest novelty of all. With a melon, the individual unit is not the entire melon, since a person does not eat a whole melon in one sitting, but rather the unit is a small slice, the measure of a single portion. Therefore the buyer may not select the entire melon, since that would be like a collection of melon units, but rather he takes slice after slice while the rest of the melon remains attached to its stalk and to the ground, and then he is exempt from ma'aser. But if he takes several portions at once, and even half a melon, this is a collection, and now the whole of it requires tithing by rabbinic law, and so the halachah is decided.
One who buys fruit while it is still attached:
Until now we have been speaking of an arrangement in which the buyer pays, picks, and by his picking the fruits become his one at a time. But if he said explicitly: I hereby make a formal acquisition on these particular fruits on your tree, and here is the money for them - it turns out that he has bought fruit attached to the tree. And as was explained earlier: one who buys an entire tree becomes like the new farmer, and is not obligated to tithe until the completion of the work. The same applies to one who buys half a tree or a portion of it, and the same applies to one who buys particular fruits while they are still attached - he is like the new farmer, and casual eating is entirely permitted to him.
And this is what the mishnah says: "Aval im amar lo b'esrim te'einim eilu, bi'shnei eshkolos eilu, bi'shnei rimonim eilu, bi'shnei avatichim eilu - ocheil k'darko u'patur, mipnei she'kanah b'mechubar la'karka" - but if he said to him, "for these twenty figs, for these two clusters, for these two pomegranates, for these two melons" - he eats in his usual manner and is exempt, because he acquired them while they were attached to the ground. Since he asked for those particular twenty figs, or those particular two clusters, two pomegranates or two melons - he acquired them while they were still on the tree, and therefore he is exempt. He eats in his usual manner and as he wishes, without a completion of the work and without bringing them into the house, for at the moment of purchase the produce was attached to the tree and entirely exempt from ma'asros, and he is like the new farmer who eats until the final completion of the work.
In summary: our mishnah deals with a buyer who pays first and then selects the fruits for himself from the tree. Since the fruits are not specified, and the buyer's picking does not constitute a completion of the work from the seller's side, he may eat without tithing - provided that no collection forms in his hand: one fig, one grape, one pomegranate seed or one slice of melon at a time. But if he made a formal acquisition on particular fruits while they were still attached to the ground, he becomes like the new farmer and eats in his usual manner, entirely exempt.