Bava Batra 126A

ומלוה שעמו, פלגי.

And as for a loan that is with the firstborn, i.e., he had borrowed money from his father, then his father died, it is uncertain whether the payment should be considered property due to the father or property possessed by him. Therefore, the firstborn and his brothers divide the additional portion.

אמר רב הונא אמר רב אסי: בכור שמיחה ⁠– מיחה.

§ With regard to the halakha that the firstborn is not entitled to a double portion of the enhancement of the property resulting from the actions of the heirs, Rav Huna says that Rav Asi says: A firstborn who protested the efforts of enhancing the property before it is divided has protested, and if the brothers use resources from the estate to enhance it against his will, he is entitled to a double portion of the enhanced value.

אמר רבה: מסתבר טעמיה דרב אסי בענבים ⁠– ובצרום, זיתים ⁠– ומסקום; אבל דרכום ⁠– לא. ורב יוסף אמר: אפילו דרכום.

Rabba said: Rav Asi’s opinion is reasonable in a case where they inherited grapes on a grapevine and the brothers harvested them against the will of the firstborn, or if they inherited olives on olive trees and the brothers harvested them, as in these cases, the produce itself did not change. But if they treaded on them, converting them into wine or oil, even if the firstborn protested their doing so, he is not entitled to a double portion. And Rav Yosef said: Even if they treaded on them, the firstborn is entitled to a double portion.

דרכום?! מעיקרא עינבי, השתא חמרא!

The Talmud asks: Why is he entitled to a double portion, according to Rav Yosef, even if they treaded on them? Since the brothers transformed the produce, as initially it was in the form of grapes and now it is wine, they have acquired it in the same manner that a thief acquires an item he stole. Therefore, the firstborn should have no share of the enhancement.