Bava Batra 147B

לתפלתו של כהן גדול.

The Talmud answers: The difference is with regard to the prayer of the High Priest, who would pray on Yom Kippur for beneficial weather, and this knowledge enabled him to formulate his prayers accordingly.

ורבא אמר רב נחמן: מתנת שכיב מרע מדרבנן בעלמא היא, שמא תטרף דעתו עליו.

The Talmud resumes the discussion with regard to the gifts of a person on his deathbed: And Rava says that Rav Naḥman says: The halakha that the gift of a person on his deathbed does not require an act of acquisition is merely by rabbinic law, and it is instituted lest he see that his will is not being carried out and he lose control of his mind due to his grief, exacerbating his physical state.

ומי אמר רב נחמן הכי?! והא אמר רב נחמן, אף על גב דאמר שמואל: המוכר שטר חוב לחבירו, וחזר ומחלו ⁠– מחול; ואפילו יורש מוחל; מודה שמואל שאם נתנו במתנת שכיב מרע, דאינו יכול למוחלו.

The Talmud asks: And did Rav Naḥman actually say this? But doesn’t Rav Naḥman say: Even though Shmuel says that with regard to one who sells a promissory note to another and then forgives the debt, the debt is forgiven and the note is nullified, and even the heir of the creditor can forgive the debt, Shmuel concedes that if the creditor gave the promissory note as the gift of a person on his deathbed, his heir cannot forgive the debt?

אי אמרת בשלמא דאורייתא, משום הכי אינו יכול למחול. אלא אי אמרת דרבנן היא, אמאי אינו יכול למחול? אינה של תורה, ועשאוה כשל תורה.

Granted, if you say that the gift of a person on his deathbed is valid by Torah law without an act of acquisition, due to that reason the heir cannot forgive the debt, as the debt was acquired by another. But if you say that it is valid by rabbinic law, why is the heir unable to forgive the debt? The Talmud replies: Rav Naḥman maintains that although this gift is not effective by Torah law, nevertheless, the rabbis made it a halakha with the force of Torah law.