"he is Mine": What is the intent of this? It is written (Devarim 15:19) "the male shall you sanctify to the L–rd your G–d." (How am I to understand this?) As sanctify it so that you receive reward, or if you sanctify it, it is sanctified, and, if not, it is not sanctified? It is, therefore, written "he is Mine" — in any event. What is the intent, then, of "the male shall you sanctify"? Sanctify it (i.e., dedicate it to the L–rd) for the sake of receiving reward.
If the First-born Already Belongs to God
Mekhilta Tractate Pischa 16:5