Matthew Poole and continuators on Numbers 3:41

41 And thou shalt take the Levites for me (I am the LORD) in stead of all the first-born among the children of Israel , and the cattle of the Levites in stead of all the firstlings among the cattle of the children of Israel.

To wit, such as are now alive of them, but those which should be born of them hereafter are otherwise disposed.

Not that they were to be taken from the Levites or to be sacrificed to God, any more than the Levites themselves were, but that they together with the Levites were to be presented before the Lord by way of acknowledgment, that the Levites might be set apart for Gods service, and their cattle for themselves as Gods ministers and for their support in Gods work.