He handed the money over to Aaron and his sons as the redemption price for the extra Israelites.”
collect five shekels for each of them (using the sanctuary shekel standard of twenty gerahs).
Moses collected the redemption money for those extra Israelites that were more than the number redeemed by the Levites.
Moses gave this redemption money to Aaron and his sons as the Lord had told him to, following the Lord's instructions.