Demai, Chapter 6, Mishnah 2. In the previous Mishnah we learned about the mekabel - a sharecropper who pays the landowner a percentage of the crop - that he does not need to separate ma'aser before handing the owner his share, and it makes no difference whether the landowner is a Jew or a non-Jew.
The two forms of tenancy:
Sharecropping (mekabel): the payment to the landowner is set as a percentage of the crop.
Renting (chocher): the payment is not a percentage, but a fixed quantity of produce.
"HaChocher sadeh min hanochri - me'aser venosein lo" - one who rents a field from a non-Jew separates ma'aser and then gives him his payment.
Our Mishnah teaches the novel ruling that one who rents a field from a non-Jewish landowner must separate ma'aser before giving him the fixed quantity he obligated himself to pay.
The reason for the ruling: a penalty
The Sages imposed a penalty on one who rents a field from a non-Jew, in order to make the arrangement difficult for him. Once renting involves separating ma'aser, the renter will no longer be interested in entering into such deals with a non-Jewish landowner. Consequently, the non-Jew will find no one to rent his field to, and he will be forced to sell it to a Jew at a low price. This achieves a benefit we seek in Eretz Yisrael: removing land from non-Jewish ownership and transferring it into Jewish hands.
Why was this ruling stated only regarding renting and not sharecropping? Because non-Jews were far more interested in a fixed-payment arrangement than in a percentage arrangement, and therefore the penalty - the obligation to separate ma'aser - was imposed specifically in the case of renting.
The view of Rabbi Yehudah:
Rabbi Yehudah adds a further case in which a person must separate the ma'aser: "Af hamekabel sedeh avosav min hanochri - me'aser venosein lo" - even one who sharecrops his ancestors' field from a non-Jew separates ma'aser and then gives him his share. Even in sharecropping, in a percentage arrangement, if the field is his ancestors' field - a field that belonged to his fathers and his fathers' fathers and was seized from them by non-Jews - and he now takes it on as a sharecropper, he must separate ma'aser.
Here too this is a penalty, designed to make things difficult for the sharecropper: since it is hard for him to give up the ma'aser and still hand over a percentage, he will make the effort to purchase the field that belonged to his ancestors. And since the field was his family's heritage, we assume that his interest in it is stronger, and he will push himself to get it out of non-Jewish hands.
In summary: in this Mishnah we learned that one who rents a field from a non-Jew - in a fixed-payment arrangement - separates ma'aser before giving the landowner his payment, as a penalty imposed by the Sages in order to make such deals difficult and to bring the non-Jew to sell his field to a Jew. This penalty was stated only regarding renting, since that was where the non-Jews' primary interest lay. Rabbi Yehudah adds that even one who sharecrops his ancestors' field from a non-Jew in a percentage arrangement separates ma'aser and then gives him his share, so that he will push himself to purchase his ancestral heritage and remove it from non-Jewish hands.