Introduction to Mitzvot 79 - 82
The Uniqueness of the First-born
In Egypt, the tenth and final plague, which led to the exodus of the Jewish people, was the slaughter of all Egyptian first-born men and animals.
HaShem guarded the homes of the Jews and their first born remained unharmed.
The Torah tells us that HaShem then claimed for Himself the first-born of the Jews and their beasts, and declared them holy.
Certain Mitzvot apply only to the first-born.
Positive Mitzvah 80
Redeeming First-born Sons
Numbers 18:15 "Nevertheless the first-born of man you must surely redeem"
Since our first-born sons were claimed by HaShem, they must be redeemed by giving five silver coins to the priest.
Fathers should fulfill this Positive Mitzvah when the baby boy is one month old. This is called the Pidyon HaBen.
If for some reason the Pidyon HaBen did not take place at that time, the first-born son must carry out the Pidyon HaBen himself.
Positive Mitzvah 81
Redeeming First-born Donkeys
Exodus 34:20 "The first-born of a donkey you shall redeem with a lamb"
The specialty of the first-born affects only one non-kosher animal - the donkey.
Since a non-kosher animal cannot be sacrificed, it must be redeemed.
Its owner is commanded to pay the priest its value or give him a lamb in exchange.