And every firstling of a donkey you shall redeem with a lamb; and if you will not redeem it, then you shall break its neck: and all the firstborn of man among your children shall you redeem.
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Ambrose of Milan
The law has established that an unclean animal shall not be a part of a sacrifice but in its place a clean animal be offered. The law orders that the offspring of an ass, which is unclean, should be changed for a sheep, which is a clean animal and suitable for sacrifice. This is the literal meaning. If one were to pursue this matter further and seek for the spiritual sense of this passage, he will discover that the ass is a laborious animal, whereas the sheep is productive. This may be interpreted to mean that labor should be exchanged for produce, since the final results of work is the produce thereof. Or we may interpret the passage in this manner: Every action or labor of yours you can make commendable by the pure and simple manner in which you perform it. .
Price. No other option is given, as the Levites were selected for the ministry. (Haydock)
The first offspring of impure animals, were to be redeemed or killed; those of the pure were to be offered in sacrifice, Numbers xviii. 15. (Philo.) Dogs, cats, poultry were to be slain, Deuteronomy xxiii. 18. (Calmet)