ולא היא דאמר רב יימר בר שלמיה לדידי מפרשא לי מיניה דאביי בין מצר ארעא דמינה פלגא ובין מצר ארעא דמינה פסיקא אי א"ל אלין מצרנהא פלגא לא א"ל אלין מצרנהא תשעה קבין:
But that is not so, as Rav Yeimar bar Shelemya said: The matter was explained to me by Abaye, as follows: Whether the seller writes with regard to the fourth boundary: The boundary of the field is the land through which the field is halved, or he writes: The boundary of the field is the land through which a plot can be set apart, if he said to the buyer: These are its boundaries, he has sold him half of the field. But if he did not say to the buyer: These are its boundaries, he has sold him only an area fit for sowing nine kav of seed.
פשיטא אמר יחלוק פלוני בנכסי פלגא תנו חלק לפלוני בנכסי מאי
§ The Gemara raises a question about a similar case: It is obvious that if one said: So-and-so should share in my property, he means to give him half of the property. If he said: Give so-and-so a portion of my property, what is the halakha? What portion of the property must he give him?
אמר רבינא בר קיסי תא שמע דתניא האומר תנו חלק לפלוני בבור סומכוס אומר אין פחות מרביע לחבית אין פחות משמינית לקדרה אין פחות מי"ב לטפיח אין פחות מששה עשר
Ravina bar Kisi said: Come and hear a proof concerning the halakha in this case, as it is taught in a baraita: With regard to one who says: Give so-and-so a portion of my cistern for his water needs, Sumakhos says: He must give him not less than one-quarter of the water in the cistern. If he qualifies his words and says: Give so-and-so a portion of my cistern’s water for his barrel, he must give him not less than one-eighth of the water. If he says: Give him a portion for his pot, he must give him not less than one-twelfth of the water. And if he says: Give him for his cup, he must give him not less than one-sixteenth of the water. In any event, this baraita indicates that the unqualified phrase: Give so-and-so a portion, should be understood to mean: Give him one-quarter.
תנו רבנן בן לוי שמכר שדה לישראל ואמר לו על מנת שמעשר ראשון שלי מעשר ראשון שלו ואם אמר לי ולבניי מת יתן לבניו
The Gemara now considers another case in which the seller withholds something for himself in a sale. The Sages taught in a baraita: With regard to a Levite who sold a field to an Israelite and said to him: I am selling you this field on the condition that the first tithe from the produce growing in the field, which must be given to a Levite, is mine, and it will be given to me every year and not to any other Levite, the first tithe is his. And if he said: I am selling you the field on the condition that the tithe will be given to me and to my sons, then if he dies, the buyer must give the tithe to his sons.
ואם אמר לו כל זמן שהשדה זו בידך מכרה וחזר ולקחה אין לו עליו כלום
But if the seller said to the buyer: This stipulation will remain in force as long as this field is in your possession, then if the buyer sold it and afterward bought it back again, the seller has no claim on him. Since the field left the buyer’s possession in the interim, the seller no longer has a claim to the tithe.
אמאי אין אדם מקנה דבר שלא בא לעולם כיון דאמר ליה על מנת שמעשר ראשון שלי שיורי שייריה למקום מעשר
The Gemara challenges the halakha taught in the baraita: Why should the seller ever have a right to the tithe after he has sold the field? After all, a person cannot transfer ownership of an object that has not yet come into the world. How, then, can the seller acquire a portion of the produce that does not yet exist? The Gemara answers that since the seller said to the buyer: I am selling you this field on the condition that the first tithe is mine, it is as if he withheld the site where the tithe is grown for himself when he sold the field, and that site already exists.
אמר ריש לקיש זאת אומרת המוכר בית לחבירו ואמר לו על מנת שדיוטא העליונה שלי דיוטא העליונה שלו
Reish Lakish said: That is to say that with regard to one who sells a house to another and says to him: I am selling you this house on the condition that the upper story [deyota] is mine, the upper story is his.