3,345. Incentivizing the Distribution of Priestly Gifts
Terumos 12:20
One may not give terumah to his vat watchman, nor a firstborn to his flock watchman, nor the gift pieces of an animal to his shepherd. If he did so, he makes a desecration unless he paid them their wages first (in which case it’s clear that the kohein isn't receiving these gifts in exchange for his labor). One Yisroel may tell another, “Here’s a sela (a denomination of coin); give terumah/a firstborn/other gifts to Kohein X, who is my grandson/my nephew/etc.” (The owner can do as he likes with such gifts, so he can accept inducement regarding how to distribute them.)
Terumos 12:21
The preceding halacha applies when the owner wants to freely give the gifts to one of two kohanim, or tithes to one of two Leviim. His friend then offers him something in exchange for giving the gift to a particular one of those two. However, the owner may not ask the recipient kohein or Levi to pay him for his right to give the gift to whomever he wishes. Similarly, terumah may not be sold as merchandise even if one buys it from a kohein and sells it to a kohein.