If a Kohen sold a field to a Yisrael and told him, "It is on condition that the Maaseros are mine for four or five years'', the Kohen may prevent him from planting a vineyard, from planting Istis (which is a plant that is exempt from Maaseros) and from making it a cane field (for the same reason).

But if he stipulates, "...the Maaseros are mine forever'', he cannot prevent him from doing those things.

A case came before R. Acha bar Ila (like the first case) and (after the buyer planted a cane field) R. Acha obligated him to give to seller 10% of the canes.

If a Yisrael sold a field to a Kohen and said, "...on condition that the Maaseros for the next 4 or 5 years are mine'', it's permitted. "...forever'', it is prohibited, as it appears as if a Kohen has appointed a Kohen. If a Kohen sold a field to a Yisrael and said, "...on condition that the Masseros for the next 4 or 5 years are yours'', it's prohibited.

Question: Why over here (Yisrael sold to Kohen) it said that it's permitted and here (Kohen sold to Yisrael) it said prohibited?

Answer: Here (Yisrael to Kohen), the field is in the Kohen's possession (and the Maaseros are his), so he can give them to the Yisrael); here (Kohen to Yisrael), the Maaseros are not his, so he cannot turn the Yisrael into a Kohen'.