Bava Batra 124A

אף מוחכרת ומושכרת ⁠– שבחא דממילא קא אתי, דלא חסרי בה מזוני.

so too in the case of a cow that was leased or rented, the baraita is referring only to a case where the enhancement came by itself, as the brothers did not lose money for its sustenance, since it was stipulated that the one who rented or leased it would provide its feed.

מני ⁠– רבי היא, דתניא: אין בכור נוטל פי שנים בשבח ששבחו נכסים לאחר מיתת אביהן. רבי אומר, אומר אני: בכור נוטל פי שנים בשבח ששבחו נכסים לאחר מיתת אביהן, אבל לא בשבח שהשביחו יתומים לאחר מיתת אביהן.

§ The Talmud continues its discussion of the baraita. In accordance with whose opinion is the baraita? It is in accordance with the opinion of R' Yehuda HaNasi, as it is taught in a baraita (Tosefta 7:4): A firstborn does not take a double portion of the enhancement of the property that occurred after the death of the sons’ father. R' Yehuda HaNasi says: I say that a firstborn does take a double portion of the enhancement of the property that occurred by itself after their father’s death, e.g., the birth of a calf, but not of the enhancement that the orphans caused after their father’s death.

ירשו שטר חוב ⁠– בכור נוטל פי שנים. יצא עליהן שטר חוב ⁠– בכור נותן פי שנים. ואם אמר: ״איני נותן, ואיני נוטל״⁠ – רשאי.

R' Yehuda HaNasi continues: Therefore, if they inherited a promissory note indicating a debt owed to their father, the firstborn takes a double portion of the money when it is collected, as this is an enhancement to the estate that came by itself. The Talmud adds: In a case where a promissory note emerged against them for their father’s debt, the firstborn gives, i.e., repays, a double portion of the debt. But if he says: I am not giving a double portion of the debt and I am not taking a double portion of the estate, he is permitted to do so, and he is exempt from paying a double portion.