Numbers 3:51
And Moses gave the money of them that were redeemed unto Aaron and to his sons, according to the word of the LORD, as the LORD commanded Moses. And Moses gave the money of them that were redeemed unto Aaron and to his sons, according to the word of the LORD, as the LORD commanded Moses.
Moses gave the money paid to redeem Israel's first-born to Aaron and to his sons, according to the word of the Lord, as the Lord commanded Moses. Even after the Levites were appointed to sanctuary service, the first-born still had to be redeemed. The money from that redemption sustained the priesthood as its work passed to the Levites.
The verse belongs to the instruction in the book of Moses prescribing the divisions of the priests and the courses of the Levites for the service of God at Jerusalem. When the returned exiles arranged the service of the dedicated temple, they set the priests in their divisions and the Levites in their courses according to that instruction. It belongs to the law that established the Aaronic priesthood for its appointed age, lasting until the type met its antitype in Christ.