We now turn to Mishnah 12 of chapter 6. The Mishnah deals with an am ha'aretz who asks a chaver to buy food items for him, and with the question of whether the chaver bears responsibility to separate maaser from the item he hands over to him.
The case: "An am ha'aretz who said to a chaver":
An am ha'aretz turns to a chaver and asks him: "Kach li agudas yarak" - buy me a bunch of vegetables, and likewise "v'kach li kikar" - and buy me a loaf of bread. The chaver is going to the store for his own needs anyway, and while he is there he buys for the am ha'aretz as well.
"Lokeach stam u'patur":
The chaver buys two items - two bunches of vegetables or two loaves of bread - one for himself and one for the am ha'aretz, without designating in advance which of them is his and which belongs to the am ha'aretz. In such a case the Mishnah rules: "Lokeach stam u'patur" - he buys them without designation and is exempt from tithing the item he hands over to the am ha'aretz.
Ordinarily the chaver does bear responsibility: anything that leaves his hands and is given to an am ha'aretz, he must make sure that maaser has been separated from it. Here, however, even though he did not identify in advance which of the two bunches of vegetables or which of the two loaves of bread was his and which belonged to the am ha'aretz, we apply the principle of 'bereirah' - the very item he ultimately gave him is retroactively established as the one that was purchased from the outset for the am ha'aretz. It turns out that the chaver never owned that vegetable or that loaf, and therefore he never had any responsibility to separate maaser from it. The am ha'aretz himself may buy it and do with it as he wishes, and hopefully he will separate maaser, but the chaver bears no responsibility, since the item was never his.
"V'im amar: sheli zu v'shel chaveri zu, v'nis'arvu":
But if at the time of purchase he designated them and said "sheli zu" - this bunch or this loaf is mine, "v'zu shel chaveri" - and this one belongs to my fellow, meaning the am ha'aretz, and afterwards "nis'arvu" - they became mixed up and he does not know which is his and which is his fellow's - then "chayav l'aser" - he is obligated to tithe. The reason: the item he hands to the am ha'aretz may well be his own, given in exchange for his fellow's, and anything of his own that leaves his hands to an am ha'aretz he must separate maaser from.
"Va'afilu hen me'ah":
This ruling applies even in a ratio of one to a hundred: the am ha'aretz bought ninety nine loaves and the chaver only a single loaf, and the loaves became mixed up. When he hands the am ha'aretz his ninety nine loaves, he bears the responsibility to make sure that maaser has been separated from each and every one of them, since any one of them might be precisely the one that belongs to him. He is therefore obligated to separate maaser from the entire quantity.
In summary: in this Mishnah we learned two situations. When the chaver buys without designation, without singling out the items, we apply 'bereirah' and establish retroactively that the item handed to the am ha'aretz was purchased for him from the outset, and therefore it never belonged to the chaver and he is exempt from tithing. But when he designated them in advance, "this one is mine and this one is my fellow's," and afterwards they became mixed up, every item he hands over might be his own, and therefore he is obligated to tithe - even if his share is one out of a hundred, in which case he must separate maaser from the entire quantity.