Demai, Chapter Six, Mishnah One
This mishnah deals with a Jew who rents a field from someone else, either a Jew, a Gentile or a Samaritan. There are two types of rental agreements referred to here. In the first type of agreement, the owner of the field shares part of the crop with the one renting and working the field. In the second type of agreement, the one renting pays the owner a fixed amount of the produce, whether the field produces a good or a bad crop.
1) One who has received a field from a Jew, or from a Gentile, or from a Samaritan [for a share in the produce], he divvies up the produce in front of them [without first separating tithes].
2) One who has hired a field from a Jew [for a fixed amount from the produce], he first gives terumah [from the rental] and then gives it to him (the field owner).
3) Rabbi Judah said: When is this so? When he pays him [the rental with produce] of the same field and of the same kind; but when he pays him with the produce of another field or of another kind, he must [also] tithe [the rental first] and then give it to him.
Section one: In this case, the owner and the renter share the produce according to a set percentage. The percentage of the produce that the renter gives to the owner never belonged to the renterfrom the outset it belonged to the field owner. Therefore, when he divides the produce with the field owner he does not need to first separate terumah or tithes.
Section two: In this case, the renter is to pay the owner a fixed amount of the produce. Since the produce first belongs to the renter and then he gives it over to the owner, the renter must first take out terumah before he gives it to the owner. However, he does not have to take out tithes as well because with regard to tithes the rabbis were lenient on the sharecropper, forcing the field owner separate them himself.
Section three: Rabbi Judah limits the above law. When the renter pays the field owner with produce from the very field that he received from him, then he can get away without separating tithes. In this case, it is almost as if the produce that he gives him actually belonged to the field owner in the first place. However, if the renter is going to pay his debt with produce that he grew in another field, then he must first separate tithes and only then pay his debt.