Buying Ready Dough from a Store for Challah Separation

There are stores that sell dough in the amount required for challah, from which challah has not been separated. Is it permissible to buy this dough and separate challah with a blessing?

If the dough was kneaded separately, it is permissible. However, if a large dough was kneaded and then divided into portions for sale, it is forbidden because the baker is obligated to separate challah, and therefore, one challah separation should cover all the dough. Now, even though there is an obligation to separate with a blessing, it causes an unnecessary blessing since it could have been exempted with one separation.

Shach, Yoreh De'ah, Siman 326, Se'if Katan 4: "Bach writes that there are those who buy from the baker a portion of a large dough, and in that portion, there is the required amount, and they separate challah from it and bless it, and thus he sells from that dough to many people, each with the required amount, and each separates from their portion and blesses it. And this is clearly a mistake because as soon as the baker kneaded the large dough, he is obligated to separate challah from it, even if he intended to sell it in small portions, less than the required amount, all the more so here, where his intention is to sell it in portions with the required amount. And since the mitzvah obligates him to separate challah, and then all the dough is exempt, how can we permit initially selling in portions and having the buyers separate challah and bless, and the baker does not separate and bless? Furthermore, since with the baker's separation of challah, all the dough is exempt, we should not sell it in portions and have each one bless separately due to an unnecessary blessing. And he writes that he received from his teachers that it is a complete prohibition, and he elaborated on this in his response. But in the response of Masa'at Binyamin, Siman 1, he writes that strictly speaking, one should distinguish between a baker and an ordinary person, but not leniently, and therefore, one should not object to women buying from a Jewish baker, and each one separates her challah separately if it is in the required amount. On the contrary, one should uphold and strengthen their custom so that each woman can easily and without trouble take challah every Friday eve, as it has already been customary to take challah according to the Jerusalem Talmud, Tractate Shabbat, the first man is the challah of the world, etc., and Eve caused him death, therefore the mitzvah of challah was given to the woman, etc., and his words are not obligatory."