Sforno on Exodus 13:14/ספורנו על שמות י״ג:י״ד

1מה זאת?, a reference to the unusual legislation of having to redeem the firstborn impure animal donkey seeing that its body could not have been sanctified due to its inherent impurity. Similarly, the need to break this animal’s neck as an alternative to redeeming it sounds peculiar to the son who is asking.

2בחוזק יד הוציאנו ה' ממצרים, due to the urgency with which the Egyptians wanted to get rid of us (12,33) we left Egypt traveling in coaches, just as the family of Yaakov had arrived in Egypt hundreds of years ago in coaches supplied by Pharaoh himself. Both the coaches materialised miraculously, as well as the many donkeys needed to transport the new found riches of the people. Seeing that the donkeys had therefore had a constructive part to play in the redemption of the Israelites, this is reflected in their not being treated like all other impure beasts. This degree of minor sanctity accorded the firstborn donkey may be redeemed in exchange for a pure animal, so that the donkey can then be used in the regular way.