Problem: Numbers 18:15–16 instructs that unclean beasts should be redeemed with money. But Exodus had commanded that they be killed. How can this discrepancy be reconciled?
Solution: God apparently instructed Moses to modify the earlier law in favor of revenues for the sanctuary. Money would be more useable than so many animals. Thus, the essence of the first law was being implemented in another way, due to the circumstances. Actually, their obedience to the first law may have occasioned the need for the second. There may have simply been too many animals given to the service of the tabernacle, when other things that money could buy were in greater demand.