Before us is Mishnah 6 in the fifth chapter of Maseches Peah, which deals with the question of which party - the seller or the buyer - may take the gifts to the poor for himself from a field that was sold, and with the prohibition against deriving benefit from the gifts to the poor improperly.
One who sells his field before the harvest:
The seller - he is permitted to take from the field he sold, since at the time of the harvest the field no longer belongs to him.
The buyer - he is forbidden to take from it, even if he is poor, because the field is his and the obligation to give rests upon him.
He sold the produce and kept the land in his own hands:
The Yerushalmi notes an additional case: a person who does not sell the field itself, but sells another only the standing produce, keeping the land for himself. In such a case neither the seller nor the buyer may take the gifts to the poor for himself, and both are forbidden to do so. And the reason is:
"Sadecha" - your field. The Torah says not to finish off the corner of the field, and this expression refers to the seller, since the rights to the land remained in his hands. He is the owner, the obligation of Peah rests upon him, and he cannot take it for himself.
"Ketzircha" - your harvest. This expression refers to the buyer, who purchased the standing produce, and the harvest is his harvest.
A condition when hiring workers:
A further matter to be careful about is that a person should not derive benefit from the gifts to the poor in an improper manner: a person may not hire workers to labor in his field on condition that the worker's son will have the right to take the leket after him. The reason is that in this way the field owner may receive cheaper wages, since the worker stipulates that his son will glean following his work. It turns out that the field owner profits at the expense of the gifts to the poor, for other poor people will not benefit from the leket, only that one poor person, and through withholding the leket from the rest of the poor the employer obtains a cheaper price.
One who interferes with the poor - robs the poor:
From here the Mishnah moves to a broad principle: one who does not allow the poor to take the leket, and even one who allows one person and does not allow another - something he has no right to do - and even one who assists one particular poor person, is considered as robbing the poor, since he is not treating them equally.
And regarding this, Mishlei says: "Al tasseg gevul olim" - do not move back the boundary of the olim. This is a delicate expression, and two explanations were given in clarifying its meaning:
It refers to Israel as a whole - those who went up out of Egypt, the ones who ascended from Egypt.
It refers to those who came down from their holdings and lost their money, whose fortune went down. And although, according to the content of the matter, it would have been fitting to say "those who go down," the verse chose a delicate and positive expression: "olim," those who go up.
In summary: In this Mishnah we learned that one who sells his field before the harvest may take from it, whereas the buyer is forbidden to do so even if he is poor; and that where one sells only the produce and keeps the land, both are forbidden, since "sadecha" refers to the seller and "ketzircha" to the buyer. We also learned the prohibition against hiring workers on condition that their son will glean after them, through which the field owner benefits from the gifts to the poor, and the principle that anyone who withholds the leket from the poor, or discriminates among them, is considered as robbing them, and concerning him it says, "Al tasseg gevul olim."