Bekhorot 11A

בהשג יד כתיב.

is written with regard to affordability, i.e., a case where one vows to give the valuation of an individual to the Temple but does not have sufficient funds to fulfill his vow. In such a case, he must pay at least a shekel. It does not apply to other matters.

אמר רב נחמן: הלכה כדברי חכמים, וכמה? אמר רב יוסף: אפילו פטרוזא בר דנקא, אמר רבא: אף אנן נמי תנינא גדול וקטן.

Rav Naḥman says: The halakha is in accordance with the statement of the Rabbis that one may redeem a firstborn donkey with a lamb that is worth even less than a shekel. And how much must the lamb be worth? Rav Yosef says: Even a lean lamb [patruza] that is worth only a ma’a [danka] may be used. Rava says: We learn in the Mishnah (9a) as well that the owner may give either a big or a small lamb, which supports Rav Yosef’s ruling.

פשיטא! מהו דתימא כולי האי לא? אי נמי, פטרוזא לא? קא משמע לן.

The Talmud asks: Isn’t it obvious that even a lean lamb worth one ma’a may be used? The baraita states that the lamb may be worth any amount. The Talmud answers: Lest you say that if the lamb is of such little value one may not redeem with it, Rav Yosef states that one may redeem with it. Alternatively, it could be claimed that a lean lamb may not be used for redemption due to its physical state. Therefore, Rav Yosef teaches us that one may redeem a firstborn donkey even with such a lamb.

רבי יהודה נשיאה הוה ליה פטר חמור, שדריה לקמיה דרבי טרפון, אמר ליה: כמה בעינא למיתב לכהן? אמר ליה: הרי אמרו: עין יפה בסלע, עין רעה בשקל, בינונית ברגיא.

R' Yehuda Nesia had a firstborn donkey. He sent it before R' Tarfon, and said to him: How much do I need to give a priest for its redemption, i.e., how much must the lamb used for its redemption be worth? R' Tarfon said to him: Didn’t the rabbis say: One who has a generous disposition redeems his firstborn donkey with a lamb worth a sela, one who has a miserly disposition redeems his donkey with a lamb worth a shekel, and one who has an intermediate disposition redeems his donkey with a lamb worth a ragya?