Bekhorot 13A

רב ששת אמר: לומר שאינו עובר עליו.

Rav Sheshet says: The mitzva of redeeming a firstborn donkey takes effect immediately after it is born, as is stated in the first baraita. The second baraita, which states that it applies only after 30 days, means to say that until that time, one does not transgress a prohibition if he has not yet performed the redemption. After 30 days, though, one transgresses a prohibition if he still has not redeemed his firstborn donkey.

מתיב רמי בר חמא: מצותו כל שלשים יום, מכאן ואילך ⁠— או פודהו או עורפו. מאי לאו מצוה לשהותו כל שלשים יום? לא, מצוה לפדותו כל שלשים יום.

Rami bar Ḥama raises an objection to Rav Sheshet from a baraita: The mitzva of a firstborn donkey applies throughout the period of 30 days from its birth; from that point forward he must either redeem it immediately or break its neck. What, does it not mean that there is a mitzva to wait to redeem it until all 30 days have passed? The Talmud answers: No, it means that there is a mitzva to redeem it throughout the period of 30 days, and after that point one has transgressed a prohibition.

אי הכי, ״מכאן ואילך או פודהו או עובר עליו״ מיבעי ליה! אלא אמר רבא: לא קשיא, הא רבי אליעזר דמקשי, הא רבנן דלא מקשי.

The Talmud asks: If so, it should have stated: From that point forward, either he redeems it or he transgresses a prohibition, rather than stating: From that point forward he must either redeem it or break its neck. Rather, Rava says: The contradiction between the two baraitot is not difficult, as this baraita, which states that the redemption is performed after 30 days, is in accordance with the opinion of R' Eliezer, who compares the redemption of a firstborn donkey to the redemption of a woman’s firstborn son, where it is stated: “From a month old should you redeem them” (Numbers 18:16). That baraita, which states that one must perform the mitzva immediately, is in accordance with the opinion of the Rabbis, who do not compare the two mitzvot of redemption.