Note to self.
There is a debate when it comes to Exodus 12-13: Do the firstborn Israelites inherently belong to God, or do they belong to him because of the Exodus? From memory, Peter Enns goes for the former, but I could be wrong about that.
Numbers 3:13 - (I have all the Levites by substitution...) "...for all the firstborn are mine. On the day that I struck down all the firstborn in the land of Egypt, I consecrated for my own all the firstborn in Israel, both of man and of beast. They shall be mine: I am YHWH."