John Calvin Commentary Exodus 34:19

"All that openeth the womb is mine; and all thy cattle that is male, the firstlings of cow and sheep." — Exodus 34:19 (ASV)

All that openeth the matrix is mine. He here defines what the offering was to be: namely, they were to redeem their children as well as the unclean animals for a price, but they were also to bring into the tabernacle whatever could be offered in sacrifice.

However, God would not have their own sons consecrated to Him, because He had chosen the tribe of Levi (as we will see elsewhere). Therefore, they were to remain free and in their own power after a monetary payment.

In the same way, unclean animals could be applied to domestic purposes, namely, after God’s price had been paid, since they belonged to Him, and He claimed them for Himself.

But if anyone did not place so high a value on a donkey or other unclean animal, the Law commanded that its neck should be broken. Otherwise, it would have been sacrilege to profit from God’s property or, what is the same, to transfer to their private use what God had decreed for Himself.