John Wesley on Numbers 3:41

Instead of the first - born - Such as are now alive of them, but those which should be born of them hereafter are otherwise disposed. Of the Levites - Not that they were to be taken from the Levites, or to be sacrificed to God, any more than the Levites themselves were; but they together with the Levites were to be presented before the Lord by way of acknowledgment, that the Levites might be set apart for God's service, and their cattle for themselves as God's ministers, and for their support in God's work.