Demai 6:2 - 6:3

החוכר מן העובד כוכבים – the Sages fined a person who is a sharecropper renting a field from an idolater for a fixed amount, that he should tithe what he gives him, in order that there should be no Israelite who is a sharecropper renting a field from an idolater for a fixed amount, that it should remain fallow ground in his hand and because of this, he should need to sell it to an Israel for a small amount of money. But when he is a tenant farmer for the field for a percentage of the crop, he was not fined because the idolater desires more that he should be a sharecropper for a fixed amount than serving as a tenant farmer for a percentage of the yield.

שדה של אבותיו – that the idolater stole it (i.e., the field) from his ancestors, and hey fined him in that he would need to tithe in order to impel/press him that he should purchase it from the idolater because it is beloved to him in that it belonged to his ancestors, he will not allow to receive it from the idolater for more than its appropriate worth, and when the tithing becomes burdensome upon him, he will purchase it. And we hold according to Rabbi Yehuda, who does not dispute the first Tanna/teacher on this.

כשם שחולקין בחולין כך חולקין בתרומה – when the owner of the field takes “one-half,” or “one-third” or “one-fourth” [as the share from the renter in a tenant farmer arrangement] from what the field produced, he takes also “one-half,” or “one-third,” or “one-fourth” from the heave-offering and the tithes that are placed upon it and gives the to any Kohen or Levi that he desires.

אף המעשרות שלהן – if the person who rents as the tenant farmer is a Kohen, all of the heave-offering is his, and if he is a Levite, all of the [First] Tithe is his.

שעל מנת כן – they went down to lease the field as a tenant farmer for a specific share/אריסות , but the Halakha is not according to Rabbi Eliezer for on this condition they went down with what they acquired.