출애굽기 34:20의 주석

ופטר חמור AND THE FIRST OFFSPRING OF AN ASS [THOU SHALT REDEEM] — but not that of any other unclean animal (Bekhorot 5b).

בכור בניך תפדה, the amount to be paid has been spelled out in Numbers 18,16.

כל בכור בניך תפדה ולא יראו פני ריקם. “You shall redeem every firstborn of your sons. They shall not appear before Me empty-handed.” If someone has the good fortune to be a firstborn this is a true distinction. To some degree he shares this distinction with G’d Himself Who is also “a first in the universe.” The offering of sacrifices prior to the sin of the golden calf was always performed exclusively by the respective firstborn of the family. This is why Yaakov was envious of his twin brother Esau who had this privilege. The wicked Esau sold his privilege, thereby displaying his contempt for service of the Lord. After the sin of the golden calf the firstborn were disqualified seeing they had participated in that sin instead of acting as true priests restraining the other Jews from worshipping the golden calf. Their function was taken over by the tribe of Levi, some of whom became priests, others performing tasks allocated to the Levites in the Book of Numbers. Although the firstborn were no longer accorded the privilege of performing priestly functions, they did retain the distinction of being firstborn, a distinction vis-a-vis people who are not firstborn. Our sages (Vayikra Rabbah 2,2) state that whenever the Torah speaks of someone being לי, “for Me,” i.e. for G’d, the meaning is that such a status is accorded permanently both in this world and in the world to come. The Jewish people who are described as being set aside for G’d with the words ואבדיל אתכם מן העמים להיות לי, “I have separated you from the other nations to belong to Me,” are an example of what we just said. Another example is Numbers 3,45 והיו לי הלוים, “the Levites will remain Mine.” The same expression occurs in connection with the earth seeing G’d says in Exodus 19,5 כי לי כל הארץ for the entire earth is Mine. The Torah also writes of the firstborn that they belong to the Lord, i.e. כי לי כל בכור. This means that the firstborn does not forfeit this status either in this life or in the world to come. Thus far the Midrash.
The commandment involving the firstborn applies exclusively to male firstborns This is what is meant when the Torah wrote כל פטר רחם בבני ישראל, “every opening of the womb amongst the sons of Israel” (Exodus 13,2). The Torah also writes וכל בכור אדם בבניך תפדה, “and every firstborn human amongst your sons you shell redeem” (verse 13 in that chapter). It is incumbent upon a firstborn to devote himself to Torah and G’d’s commandments more than other people who do not share this distinction with him. His father must perform the duty to redeem him as this commandment devolves in the first instance on the father. Should the father have failed to perform this redemption the firstborn son must redeem himself when he comes of age (Kidushin 29). If the redemption is neglected both by his father and himself he is subject to being punished for our sages write that the reason that this legislation is written in connection with the duty to perform the pilgrimage to the Temple is to remind people to redeem the firstborn as the firstborn who have been redeemed will experience the privilege of welcoming the Shechinah and will live to see the Temple rebuilt. Those who have not been redeemed will not see the rebuilding of the Temple.