Charles Ellicott Commentary Numbers 3:45

"Take the Levites instead of all the first-born among the children of Israel, and the cattle of the Levites instead of their cattle; and the Levites shall be mine: I am Jehovah." — Numbers 3:45 (ASV)

And the cattle of the Levites instead of their cattle. —There appears to have been no numbering of the cattle. Had it been otherwise, an argument might have been used in support of the prospective reference of the command to number the firstborn, derived from the fact that it would have been impossible to ascertain the number of firstborn among the cattle. It appears, however, that the whole of the cattle of the Levites was given in redemption of the firstborn of all the cattle of the other tribes.