In Sefer Vayikra (7:9), the Torah teaches that it is the kohen who offers the minhah who is given the opportunity to eat it. In the following passage (Vayikra 7:10) it appears that all of the kohanim share equally in the meal-offerings. The baraitathat appears on today’s daf explains that the share belonging to the kohanim is divided amongst all of the kohanim and cannot be traded between them. That is to say, a kohen cannot exchange his share in a korban minhah for a share in an animal sacrifice of equal value. The baraita continues and derives from different words in the pasuk (=verse) that neither can it be traded for a different minhah portion, nor for a sacrifice from a fowl, nor can a sacrifice from fowl be traded for an animal sacrifice, and so on.
Rashi explains that the situation described in the baraita is where one kohen offers to make such a trade with a fellow kohen. Later commentaries discuss the reason for this prohibition – is it because this appears to be a business transaction and it belittles the holiness of the sacrifices, or is it because the kohanim do not actually own the portions of the sacrifices, since they are effectively “eating from the Heavenly table.”
One of the questions raised by the later commentaries is why the baraita needs to find a separate source in the pasuk to prohibit each exchange of offerings. Once we determine that such exchanges are forbidden, it would seem logical that the prohibition applies to all sacrifices. The Hazon Ish explains that since the division of the offerings is based on an agreement among the kohanim, we might have thought that similar sacrifices might possibly be interchangeable, assuming that the two kohanim agreed to the switch. The baraita is emphasizing that such exchanges are forbidden under all circumstances.