Bo: Pesikta Zutrata: Mitzvah 11 – Concept 277
“Every first issue donkey you shall redeem with a lamb or kid.” (Exodus 13:13) We are commanded to redeem the first-born donkey by giving a lamb to the Cohen. (Rambam, Hilchot Bikkurim)
This legislation is restricted to the donkey, the only impure domestic animal whose first-born male issue is not of secular status.
Pesikta Zutrata on our verse explains that the reason is that the Egyptians have been compared to donkeys by the prophet Ezekiel, who wrote, “Their flesh is the flesh of donkeys.” (Ezekiel 23:20)
An additional reason is the tradition that even the poorest of the Jews left Egypt with ninety donkeys’ loads of silver and gold. To commemorate the immense wealth that Israel took out of Egypt the Torah legislated that the beasts that carried that wealth be treated as in a class by themselves.
The first-born males of donkeys needed to be redeemed before they could be used for profane purposes.
Equating donkeys with the Egyptians reminds one of the plague of the dying of the first-born Egyptians.