Mishnayos Peah Perek 5 Mishnah 2

שבולת שבקציר – he left a remnant in his harvesting one sheaf that he did not reap, and the top of that sheaf touches/reaches the standing corn, if that sheaf is cut with the standing corn, it belongs to the owner of the house, as the standing corn saves it and we don’t call it (Deuteronomy 24:19): “do not turn back to get it…”

מעשר שבולת אחת ונותן לו – since he is is obligated to give him tithed things like that which was a sheaf of gleanings which is exempt from tithes, and even though that they had yet been shaped into an even pile, the title of tithe applies upon it such as First Tithe in which the sheaves had been advanced hat the name of Tithe fell upon hem, even though they had yet formed an even pile. And how does he do this? He brings two sheaves from this pile that had been combined in the sheaves of the gleaning, and says about one of them if this gleaning is good, and if not, it is from the set tithes of the second, and he gives him the first. And in the Jerusalem Talmud, an objection is raised, for we suspect lest that this second time when he established it for tithes of this gleaning, since it is exempt from Tithes, the Tithes do not establish it, for we don’t’ separate from that which is exempt for what is liable. And we respond that the person who takes a third sheaf and says if the second one that I had established it is for Tithes is for gleaning, and it cannot be for tithes, the third is tithed for the first.

א"ר אליעזר וכי היאך העני הזה מחליף דבר שלא בא ברשותו – Rabbi Eliezer to the words of the Rabbis said to them that, that you who dispute me regarding a rich person who gleans Peah for a poor person, for I say that he should take possession and you stated that he should not take possession, how can this poor person exchange a matter that has not come into his possession? For surely, the owner of the field is not able to make assignment to him the gleanings according to their words, and it is found that this gleaning does not come into the possession of the poor person, other than according to their words, he can assign the poor person the entire pile on the condition to return it, for a gift given on the condition to return it is called a gift, and it is found that that sheaf which is gleanings comes to the hand of the poor person and he can exchange it for another. But the Rabbis state here that they did what they cannot assign as one who assigns it and we consider it as that sheaf of gleanings as if it comes into the hands of the poor person, even though that he did not merit it, for regarding this he can exchange it for another. And the Halakha is according to the Sages.