Peah, Chapter 5, Mishnah 2. This mishnah deals with two separate topics: the first is a dispute over whether the law of leket applies to grain still attached to the ground; the second is how to act when a stalk of leket becomes mixed into the householder's grain. In fact, in the first case of the mishnah the question is whether we are dealing with leket at all, or perhaps with shichechah.
"Shibolet SheBaKatzir":
The words of the mishnah: "Shibolet shebakatzir vero'shah magia lakamah - im niktzeres im hakamah harei hi shel ba'al habayis, ve'im lav harei hi shel aniyim" - a stalk among the harvested grain whose head reaches the standing grain: if it can be reaped together with the standing grain, it belongs to the householder, and if not, it belongs to the poor. The case is a single stalk lying among the harvested grain, whose head, when bent over, reaches the standing grain that has not yet been cut. If it is close enough that it can be reaped together with the standing grain in one stroke, it belongs to the householder; and if not, it belongs to the poor.
And what exactly is the mishnah discussing?
Some understand that this is not a law of leket at all but a law of shichechah, since there is no leket in grain still attached to the ground: leket is what falls during the reaping. Accordingly, the case is a stalk that was forgotten, and if it can still be reaped together with the rest of the grain that was not forgotten, it is not shichechah; and if not, it is shichechah and belongs to the poor.
Others understand that the law of leket applies even to something still attached to the ground. If the stalk was left lying as part of the harvest, it is leket and belongs to the poor, provided that it is separated from the standing grain to the point that it cannot be reaped along with it.
A stalk of leket that became mixed into the stack:
The next halachah deals with a mixture: "Shibolet shel leket shenis'arvah bagadish" - a stalk of leket that became mixed into the pile of grain that the householder had already harvested. Here we are explicitly dealing with leket, since the mishnah says so. And there are two problems here:
It is no longer possible to identify the stalk of leket, so how can it be given to the poor person?
Leket is exempt from Terumah and tithes, while the householder's grain is obligated. And when he gives the poor person a stalk of his own, perhaps that is the stalk of leket, and if not, he is exchanging it for a stalk of his own that is obligated in tithes. How, then, may he give him grain from which tithes were not separated?
Therefore the mishnah says: "Me'aser shibolet achas nosnin lo" - he tithes one stalk and gives it to him. And how is this done? Since it is not known whether this is the exempt leket, he takes two stalks and makes a condition regarding them:
About the first he says: if this is leket, it belongs to the poor; and if it is not leket, let its tithes be in the second.
And about the second he says the reverse: if this is leket, it belongs to the poor; and if not, let its tithes be in the first.
One of them he gives to the poor person, and the other will be the tithe. This is how the Yerushalmi explains the procedure that is required.
Rabbi Eliezer's question:
Rabbi Eliezer said: "Vechi hei'ach he'ani hazeh machlif davar shelo ba birshuso" - how can one exchange, on behalf of the poor person, something that never came into his possession and that he never acquired ownership of? The leket is not inherently his, and it could belong to any other poor person. How, then, can we say about the stalk of leket that it belongs to this particular poor person, and exchange it for another stalk?
The Yerushalmi notes that Rabbi Eliezer's own opinion, as we saw in the previous chapter, is that even a wealthy person - and even the farmer who is the householder himself - can acquire on behalf of the poor person without his knowledge. Rather, his question was posed according to the view of the Sages, who hold that a wealthy person cannot acquire leket on behalf of the poor person; and according to their view, how can this be done?
Rabbi Eliezer therefore proposes a different approach: he transfers ownership of the entire stack to the poor person, making him the owner of the whole pile of grain as a gift given on condition that it be returned, and then he tithes one stalk and gives it to him in place of his leket.
And the Sages disagree and say: although ordinarily a wealthy person cannot acquire leket on behalf of a poor person, in this case they instituted the acquisition for the benefit of the poor person, and it is as though he is the owner of the leket that became mixed in, and therefore it can be exchanged for another stalk.