Hilchos Bikkurim 11:8
If the kohein wants to give the redemption money back to the father, he may do so, but the father may not give him the money with the intention that it be returned. If the father had such an intention and the kohein returned the money to him, the son is not redeemed. Rather, he must give him the money as a gift with no strings attached. Later, if the kohein wants to give it back, he may do so. Similarly, if the father gives the money to the kohein as a gift on the condition that it be returned (which is halachically effective), the son is redeemed.
Hilchos Bikkurim 11:9
It’s clear that Kohanim and Leviim are exempt from redeeming their firstborn sons. Since they served as the redemption for the firstborn Yisroelim in the wilderness (Numbers 3:45), it should go without saying they themselves do not require redemption.