Reference for Bava Batra 314:4

הא מני רבי מאיר היא דאמר אדם מקנה דבר שלא בא לעולם

property [only].<span class="x" onmousemove="('comment',' B.M. 14b. ');">¹³</span> Now, if it is assumed, that where [a bond of indebtedness contained the entry]. 'That I may acquire'. [and] the debtor bought [land] and sold [it, or where the bond contained the entry]. 'That I may acquire'. [and] he bought [land] and transmitted [it] as an inheritance, [that land is] not mortgaged [to the creditor], why does the creditor seize the improvement[s]?<span class="x" onmousemove="('comment',' The improvements, surely, took place after the loan was made. ');">¹⁴</span> — This [may] represent the view of<span class="x" onmousemove="('comment',' V. supra p. 685. n. 5. ');">¹⁵</span> R. Meir who holds [that] a person may transfer possession of something that is not [yet] in the world.

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