John Gill Commentary Numbers 3:49

"And Moses took the redemption-money from them that were over and above them that were redeemed by the Levites;" — Numbers 3:49 (ASV)

And Moses took the redemption money
Of five shekels per head:

of them that were over and above redeemed by the Levites ;
or were more than those redeemed by them. A Levite redeemed a firstborn, or freed him from the redemption price, being taken in lieu of him: 22,000 Levites were answerable to 22,000 firstborn of Israel; but as there were no more Levites than the above number, there remained two hundred seventy three firstborn to be redeemed by money, and it was the redemption money of these Moses took.