Reference for Bava Batra 109:5
אמר רב אשי אמר לי הונא בר נתן קשי בה אמימר א"כ בטלת ירושת בנו הבכור דהוה ליה ראוי ואין הבכור נוטל בראוי כבמוחזק
R. Ashi said: Huna b. Nathan told me that Amemar found it difficult [to accept this view] because if this was so it would leave no room for the double portion to which a firstborn is entitled in an inheritance,<span class="x" onmousemove="('comment',' Deut. XXI, 17. ');"><sup>7</sup></span> since all [bequeathed] property would in this way become 'prospective',<span class="x" onmousemove="('comment',' Since the whole of a man's property was liable to be seized by the Government on account of his poll tax, it was not actually his at the time of death, but was due to become his when he should have paid his tax. The Rabbinical rule was that the firstborn received a double portion only of the actual assets, not of those which were due to accrue later. V. infra 119a ');"><sup>8</sup></span>