Hilchos Bikkurim 9:9

The previous halacha (that kohanim need not give the gifts) applies when a person slaughters for himself. The rule is different if a kohein is a butcher, who slaughters and sells animals in the market. In such a case, he is given two or three weeks. After that, the gifts are taken from him and given to other kohanim (since he’s slaughtering for others, who are obligated in the gifts). If the kohein has his own butcher shop, we don’t wait at all. Rather, the gifts are taken from him immediately. If he refuses to give them, he is excommunicated until he complies.

Hilchos Bikkurim 9:10

If someone slaughters an animal for a kohein or a non-Jew, he need not give the gifts. One who jointly owns an animal with a kohein must mark his share so that the gifts will fall in the kohein’s share. If he fails to do so, he must give the gifts because his partnership with the kohein is not common knowledge (so people will suspect that he simply hasn’t given the gifts). Therefore, if a kohein was with him in the butcher shop and negotiating with him, he need not mark his share. If a Jew and a non-Jew jointly own an animal, the Jew need not mark his share. The is because of a presumption that the non-Jewish partner will tell people of his partnership with the Jew, even if the Jew’s not there at the time of the sale.