Hilchos Bikkurim 9:7

One must take the gifts for kohanim from an animal that belongs to partners since Deut. 18:3: “anyone who slaughters….”

Hilchos Bikkurim 9:8

If someone buys an animal using Sabbatical-year produce, he must give the gifts to a kohein. Kohanim and Leviim need not give the gifts as per Deut. 18:3: “from the people.” There is a doubt as to whether “the people” includes Leviim, so the gifts aren’t taken from them (since the burden of proof would be on the kohein to prove that the Levi must give them). However, if a kohein took the gifts, he need not return them (because the Levi can’t prove that he doesn’t owe them!).