Reference for Bava Metzia 32:4
והאלהים אמר רב אפי' בשדה זו מכדי רב כמאן אמרה לשמעתי' כר"מ דאמר אדם מקנה דבר שלא בא לעולם
But — by God! Rab himself did maintain that even when [the seller says] 'this field' [the sale is valid], seeing that Rab stated his law in accordance with [the view of] R. Meir, who said that a man may convey [to another person] a thing which has not yet come into existence, as it has been taught: If one says to a woman: Be betrothed to me after I shall become a proselyte, [or,] after thou shalt become a proselyte, [or,] after I shall be set free, [or,] after thou shalt be set free, [or,] after thy husband will have died, [or,] after thy brother-in-law will have given thee <i>halizah</i>,<span class="x" onmousemove="('comment',' V. Glos. ');"><sup>4</sup></span>