Bekhorot 4B

ולדורות מנלן? אמר קרא: ״והיו לי הלוים״, ״והיו״⁠ — בהווייתן יהו.

The Talmud asks: And from where do we derive that the priests and Levites are exempt from their offspring being counted a firstborn for all generations? The Talmud answers that the verse states: “And the Levites should be Mine” (Numbers 3:12). The term “should be” indicates that they should be, i.e., should remain, in their current state of sanctity. Just as firstborn status did not apply to the priests or the Levites in the wilderness, it does not apply to priests or Levites in subsequent generations.

וממאי דבשה? אמר רב חסדא: נאמר כסף לדורות, ונאמר שה לדורות.

§ Rav Safra (4a) referred to the fact that the sanctity of the firstborn donkeys of the Israelites was abrogated by the lambs of the Levites. The Talmud asks: From where is it derived that this abrogation was accomplished with a lamb? Rav Ḥisda says: The word “silver” is stated with regard to the redemption of a woman’s firstborn son for later generations in the verse: “And their redemption money…should be, according to your valuation, 5 shekels of silver” (Numbers 18:16). And in addition, the word “lamb” is stated with regard to the redemption of a firstborn donkey for later generations in the verse: “And every firstborn donkey you should redeem with a lamb” (Exodus 13:13).

מה כסף האמור לדורות ⁠— בו פדו לדורות, בו פדו לאותה שעה, אף שה האמור לדורות ⁠— בו פדו לדורות, בו פדו באותה שעה.

Therefore, one can derive that just as concerning silver, which was mentioned with regard to later generations, the Israelites redeemed with it in later generations and they also redeemed with it at that time, as the surplus firstborn in the wilderness were redeemed with 5 shekels (see Numbers 3:47), so too concerning the lamb, which was mentioned with regard to later generations, they redeemed with it in later generations, and they redeemed with it at that time.