Kommentar zu Schemot 13:13

פטר חמור AND EVERY FIRST OFFSPRING OF AN ASS — but not the first offspring of other unclean animals. This exception is the enactment of Scripture, because the firstborn of the Egyptians are compared to asses (Ezekiel 23:20; Bereishit Rabbah 96); and a further reason for this exception is because they (the asses) were of assistance to the Israelites when they left Egypt, for there was not a single Israelite who did not take with him from Egypt several asses laden with the silver and gold of Egypt. (Bekhorot 5b).

תפדה, after the Levites had been sanctified to take the place of the firstborn who had participated in the sin of the golden calf, in the second year after the Tabernacle had been erected and inaugurated. At that time you will take 5 shekalim per head (Numbers 3,47) to effect this exchange. It included Levites of one month and up (males) Numbers 18,16)

וכל פטר חמור, “and every firstborn donkey, etc.” This legislation is restricted to the donkey, the only impure domestic animal whose firstborn male issue is not of secular status (as opposed to horses and camels, also widely considered as domestic beasts). Pessikta Zutrata on our verse explains that the reason is that the Egyptians have been compared to donkeys by the prophet Ezekiel (23,20) who wrote “their flesh (male organ) is the flesh of donkeys.” An additional reason is the tradition that even the poorest of the Israelites left Egypt with ninety donkeys’ loads of silver and gold. To commemorate the immense wealth the Israelites took out of Egypt the Torah legislated that the beasts which carried that wealth be treated as in a class by themselves. The firstborn males of such donkeys needed to be redeemed before they could be used for profane purposes. Equating donkeys with Egyptians reminds one of the plague of the dying of the firstborn Egyptians.