Bava Batra

This, however, is not so, because R. Yemar b. Shelemiah has said: Abaye has himself explained to me that whether he writes, 'The boundary [of the field] is the field from which half has been cut off,' or 'The boundary [of the field] is the field from which a piece is cut off,' if he adds the words, 'these are its boundaries', [then he sells him] half,  and if he does not add the words 'these are its boundaries', [then he sells him] nine kabs. We take it for granted that if a man says, Let so-and-so share  my property, [he is to receive] a half. If he says, Give so-and-so a share in my property, what is to be done?  — Rabina b. Kisi said, Come and hear: it has been taught: If a man says, Give so-and-so a share in a cistern, Symmachus  says that he is to receive not less than a quarter.  [If the man says], Give him a share [in the cistern] for his pail,  he is to receive not less than an eighth. [If he says, Give him a share] for his pot,  he is to receive not less than a twelfth. [If he says, Give him a share] for his drinking cup,  he is to receive not less than a sixteenth. Our Rabbis taught: If a Levite sells a field to any [ordinary] Israelite  with the stipulation that the first tithe  therefrom is to be given to him,  the first tithe from it must be given to him. If he stipulated that it was to be given to him and to his sons and he then died, it is to be given to his sons. If the stipulation is, 'as long as this field is in your possession,' and he [the purchaser] sells it and then buys it again, the Levite has no claim on him. How can [all] this be, seeing that a man cannot transfer to another possession of something that does not yet exist?  — Since the Levite stipulated that the first tithe should be given to him, he in effect reserved to himself the area of the tithe.  Resh Lakish said: This shows that if a man sells an apartment to another with the stipulation that the top layer  is still to belong to him, the top layer belongs to him.