Numbers 18:17 - Exposition

But the firstling of a cow, &c.; Only those things which were not available for sacrifice could be redeemed; the rest must be offered to him that claimed them. The first-born of men belonged partially to both classes: on the one hand, they could not be sacrificed, and therefore were redeemed with money; on the other hand, they could be dedicated (being clean), and therefore had been exchanged for the Levites.