John Gill Commentary Numbers 3:51

"and Moses gave the redemption-money unto Aaron and to his sons, according to the word of Jehovah, as Jehovah commanded Moses." — Numbers 3:51 (ASV)

And Moses gave the money of them that were redeemed
That were redeemed this way, and not by Levites, as in (Numbers 3:49) , but by money, paying five shekels a head:

unto Aaron and his sons ;
to whom the Levites were given; and this money, as a recompence for the deficiency of the number of them, to answer to the firstborn exchanged for them:

according to the word of the Lord ;
(Numbers 3:48) ;

as the Lord commanded Moses ;
so did he, being a faithful servant in all things in the house of God; he did not convert it to his own use, or to any other use than to what God had appointed it.