Hilchos Bikkurim 9:21

Let’s say that there’s a kohein whose friends regularly give him the gifts. If he wants to give them to a friend who is a Yisroel, he may do so, even if he never physically took possession of the gifts. Therefore, the kohein’s friends can slaughter animals and give the gifts directly to the Yisroel who acquired them. This is the case when the Yisroel has financial need and cannot purchase meat, plus the kohein transferring the gifts is his friend. However, if the kohein is the Yisroel’s attendant or employee, he can only assign him the gifts after he has taken possession of them. This is done in order to avoid the kohein being pressured to act against his will.

Hilchos Bikkurim 9:22

A kohein may not grab the gifts; he shouldn’t even ask for them! Rather, if he is given them in a dignified manner, he can accept them. If there are many kohanim at the butchery, the shy ones tend to stand back while the greedier ones take the gifts. Even if a kohein is shy, if he’s not known to be a kohein, he should take the gifts so that people will know. The kohanim must eat the gifts roasted, with mustard. This is because the gifts are given “to distinguish them” (see Exodus 29:29). Therefore, the gifts must be eaten in the manner of royalty.