[If] one makes an exchange with the poor —- with respect to his, he is exempt; but with respect to that of the poor, he is liable.  [If] two people contracted for a field as sharecroppers, this one gives to that one his share of maasar ani, and that one gives to this one his share of maasar ani.  [If] one contracts to harvest a field, he is forbidden [to take its] leket, shich'chah, peah, or maasar ani. Said R' Yehudah: When is this? When he contracts from him for a half, a third, or a quarter; but if [the owner] said to him: “A third of what you harvest shall be yours” —- he is permitted to take leket, shich'chah, and peah, but he is forbidden to take maasar ani.

[If] someone sells his field, the seller is permitted, but the buyer is forbidden.  A person may not hire workers on condition that his son shall glean behind him. [If] one does not allow the poor to glean, or [if] he allows one and not another, or [if] he assists one of them, he is robbing the poor. About this it is said: Move not the landmark of them that came up (Proverbs 22:28; 23:10).