Challah, Chapter 1, Mishnah 7. Our mishnah presents two laws concerning a baker: the first about a leavening agent made in order to be divided up, and the second about flour that women gave to a baker.
"Nachtom she'asah se'or lechaleik":
A baker who made a se'or - a dough intended to serve as a leavening agent for other loaves - and his intention is to divide it up and sell the small pieces to different people, so that no single piece he distributes will contain the amount that obligates in challah.
Even so, the mishnah rules: "chayav b'challah" - it is obligated in challah. Although he intends to divide it into small pieces that do not contain the measure for challah, the dough is obligated in challah, because it is possible that he will not find buyers and will end up baking the entire quantity himself as one loaf of se'or. And since it may turn out that he is left with a single loaf containing the measure for challah, it is obligated in challah in any case, even though he plans to sell it.
Women who gave flour to a baker to make se'or for them:
Women who gave flour to a baker so that he would make se'or from that flour through his own process, and no one of them has the required measure - that is, none of them has the measure for challah on her own, and only when all of them are combined together is there the measure for challah - it is exempt from challah.
The reason: for the obligation of challah to take effect, one person must own a complete measure of challah. Here, none of them owns a measure for challah, and therefore it is exempt from challah.
In summary: se'or that the baker made in order to divide it up is obligated in challah, since it may remain in his possession and be baked as a single loaf containing the required measure; whereas flour belonging to several women that was handed over to a baker, where none of them has the required measure and only their combination reaches it, is exempt from challah, because no single person owns a measure of challah.