Source: Michael Fishbane's Biblical Interpretation in Ancient Israel (New York: Oxford, 1988) 185.
Ezekiel 20:25-26 states:
"Moreover I gave them statutes that were not good and ordinances by which they could not live. I defiled them through their very gifts, in their offering up all their firstborn, in order that I might horrify them, so that they might know that I am the LORD" (NRSV).
According to Fishbane, many scholars believe this passage is saying that God gave laws that could be miscontrued as commanding child sacrifice.
Are there such laws in the Torah? Fishbane cites Exodus 13:13 and 34:20:
Exodus 13:13: But every firstborn donkey you shall redeem with a sheep; if you do not redeem it, you must break its neck. Every firstborn male among your children you shall redeem.
Exodus 34:20: The firstborn of a donkey you shall redeem with a lamb, or if you will not redeem it you shall break its neck. All the firstborn of your sons you shall redeem. No one shall appear before me empty-handed.
For Fishbane, the word translated "you shall redeem" can also be rendered "you may redeem." After all, the redemption of the donkey is optional, right? So why not the monetary redemption of a human firstborn? According to Fishbane, there were Israelites who decided to go above and beyond the call of duty, showing God they were serious by sacrificing their firstborn.
Fishbane speculates that Numbers 18:15 was a response to such a horrible interpretation. Vv 15-16 affirm:
"The first issue of the womb of all creatures, human and animal, which is offered to the LORD, shall be yours; but the firstborn of human beings you shall redeem, and the firstborn of unclean animals you shall redeem. Their redemption price, reckoned from one month of age, you shall fix at five shekels of silver, according to the shekel of the sanctuary (that is, twenty gerahs)."
According to Fishbane, Numbers 18:15 is more emphatic in Hebrew, clearly stating that the Israelites must redeem their firstborn rather than sacrificing them.
Ezekiel 20 may be referring to the law in Exodus 34:20, but I doubt it has Exodus 13 in mind. The reason is that Ezekiel 20 says that God gave Israel the bad laws after she trangressed the Sabbath and worshipped false gods. Exodus 34 occurs after Israel's transgression of the Sabbath in Exodus 16, and the golden calf incident in Exodus 32.
At the same time, one could argue that Exodus 13 was inserted into the Exodus story at a later time. If that is the case, then its place in the story is not exactly relevant.