Exodus 34:20 But the firstling of a donkey thou shall redeem with a lamb:

Exodus 34:20 But the firstling of a donkey thou shall redeem with a lamb: and if thou redeem him not, then shall thou break his neck.

Every firstborn creature must be the Lord’s, but since the donkey was unclean, it could not be presented in sacrifice. What then? Should it be allowed to go free from the universal law? By no means. God admits of no exceptions. The donkey has to pay its due, but He will not accept it; He will not abate the claim, but yet He cannot be pleased with the victim. No way of escape remained but redemption-the creature must be saved by the substitution of a lamb in its place; or if not redeemed, it must die. My soul, here is a lesson for you. That unclean animal is yourself; thou art justly the property of the Lord who made you and preserves you, but you are so sinful that God will not, cannot, accept you; and it has come to this, the Lamb of God must stand in your place, or thou must die eternally. Let all the world know of your gratitude to that spotless Lamb who has already bled for you, and so redeemed you from the fatal curse of the law. Must it not sometimes have been a question with the Israelite which should die, the donkey or the lamb? Would not the good man pause to estimate and compare? Assuredly there was no comparison between the value of the soul of man and the life of the Lord Jesus, and yet the Lamb dies, and man the donkey is spared. My soul, admire the boundless love of God to you and others of the human race. Worms are bought with the blood of the Son of the Highest! Dust and ashes redeemed with a price far above silver and gold! What a damnation had been mine had not plenteous redemption  been found! The breaking of the neck of the donkey was but a momentary penalty, but who shall measure the wrath to come to which no limit can be imagined? Inestimably dear is the glorious Lamb who has redeemed us from such a doom.