Reference for Bava Batra 290:2
נגבית בב"ד מאי טעמא כמלוה דמיא ואין בה משום רבית דלאו אדעתא דהכי יהב ליה ואין השביעית משמטתה דלא קרינא ביה לא יגוש
cannot be applied to it.<span class="x" onmousemove="('comment',' Since it cannot be exacted at the Sabbatical year, reciprocation not being due until the groomsman celebrates his marriage. (Cf. Mak. 3b). ');"><sup>7</sup></span> 'And the firstborn does not receive a double portion' — because it is prospective.<span class="x" onmousemove="('comment',' The reciprocated gift was never in the possession of the first-born's father; and all he inherited was only a claim for the future. ');"><sup>8</sup></span> and a firstborn does not receive [a double portion] in prospective [property] as in that which was in [his father's] possession [at the time of his death]. R. Kahana said, [This is] the rule of groomsmanship: [If] he<span class="x" onmousemove="('comment',' The man who has to reciprocate the wedding gift. ');"><sup>9</sup></span> was In town,<span class="x" onmousemove="('comment',' When his shoshbin celebrated his own marriage. ');"><sup>10</sup></span> he should have come.<span class="x" onmousemove="('comment',' With the gift. And since he did not, it may be claimed through a court of law. ');"><sup>11</sup></span>