(Mishnah): If one sells tree trunks from his field, each buyer gives Peah from the trunks that he bought.

(R. Yehuda): When is this? When the owner of the field left nothing for himself; but if he did, he gives Peah for everything.

(Gemara) Question: The Mishnah only seems to teach this law (of R. Yehuda that the seller gives Peah) if he began the harvesting (as the obligation falls on him); but if not, would the seller also give Peah for everything?

Suggestion (Mishnah in Chulin): If a person bought the shearings of his friend's sheep, if the seller withheld some, the seller is obligated in Reishis HaGez (the first shearings of wool of a flock of sheep); if not, the buyer is obligated.

(R. Yirmiyah citing R. Yochanan): That Mishnah, that does not differentiate in whether or not the owner began shearing, follows the opinion of R. Yehuda - here also, we do not differentiate in whether or not the owner began harvesting (as long as he left something for himself).

Question: What is the reasoning of R. Yehuda? Is it because the obligation is always on the (last remaining) standing crops or is it because the seller (automatically) sells in a way that retains the obligation for himself?

Suggestion (that Mishnah in Chulin): If a person bought the shearings of his friend's sheep, if the seller withheld some, the seller is obligated in Reishis HaGez; if not, the buyer is obligated. It was about this case that R. Yirmiyah quoted R. Yochanan as saying that the Mishnah is R. Yehuda.

Conclusion of answer: There (shearings), could one say that the obligation remains on the last wool that is still attached?! Rather, it is because the seller retains the obligation for himself. Similarly here, in Peah, the seller retains the obligation for himself.

Question: What is the practical difference between these two reasons?

Answer #1: If the buyer gave Peah - if you say that the obligation remains on that which is still attached, it is valid and the seller does not need to give; if it is because the seller retains the obligation, the buyer can now claim money back from the seller.

Answer #2: If the portion of the seller became burned - if you say that the obligation remains on that which is still attached, if it burned, the obligation is gone; if it is because the seller retains the obligation, if it burned, the buyer must give Peah from his produce and can then claim money back from the seller.