Child Sacrifice, Part 3

Source: Michael Fishbane's Biblical Interpretation in Ancient Israel (New York: Oxford, 1988) 186-187.

Does the Torah command sacrifice of the firstborn? We saw that Exodus 22:29-30 commands the Israelites to give their firstborn to God, but does that necessarily mean child sacrifice?

According to Fishbane, there is another way to interpret that passage. In I Samuel 1:11, we see that Hannah gave her child to God when she devoted him to the service of the temple. Consequently, when God told Israelites to give him their firstborn children, he may have meant that they should give them to the sanctuary: to do lay menial work there. In time, however, that became unfeasible, so there came the rule that parents could monetarily redeem their firstborn children.

A passage that supports Fishbane's interpretation is Numbers 3:12: "I hereby accept the Levites from among the Israelites as substitutes for all the firstborn that open the womb among the Israelites. The Levites shall be mine" (NRSV). The Levites take the place of the firstborn by doing menial service in the temple. Not suprisingly, some argue that this is what the firstborn did before God picked the Levites: the firstborn were priests who took care of the sanctuary.

Do I have any problems with this view? I can't exactly disprove it, but it seems to go against some of what Fishbane said elsewhere, which I discuss in Child Sacrifice, Part 2: Exodus 13:13 and 34:20 tells the Israelites that they may redeem their children, whereas Numbers 18:15 tells them they must. The "must" command makes sense if it's seeking to correct any sense in the Exodus passages that God accepts child sacrifices. So there presumably were people who believed that giving a child to God meant sending him up in smoke.

Also, I wonder what to say about the chronology of the narrative. Numbers 18 commands the redemption of children while also referring to the Levites as priests. But the Levites serving as priests is supposed to take care of the firstborn issue, so why do the Israelites still need to redeem their kids monetarily? Is this a contradiction in Numbers?