Responsa for Bava Batra 247:4

מאי טעמייהו דרבנן אמר קרא (דברים כא, יז) לתת לו פי שנים מתנה קרייה רחמנא מה מתנה עד דמטיא לידיה אף חלק בכורה עד דמטיא לידיה

as a gift [does not become his]<span class="x" onmousemove="('comment',' The recipient's with the power to give it away. ');">¹⁰</span> until it comes into his possession,<span class="x" onmousemove="('comment',' Lit., 'to his hand'. ');">¹¹</span> so the portion of the birthright [does not become his] until it comes into his [father's] possession.<span class="x" onmousemove="('comment',' I.e., the father cannot claim it as his, entitling him to transmit it to the firstborn, until it actually comes into his possession. ');">¹²</span>

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