Tosefta do Gittin 3:11

The landowner may not separate terumah from tithes and [thereafter] obtain permission from the Levite, nor [take] the shankbone, the jawbone or the rough-stomach [of an animal, see Deut. 18:3] and [thereafter] obtain permission from the Kohen, but he may make them a loan in order for it to count [as terumah or priestly gifts, see Gitt. 3:8]

A Yisrael who sold his field to a Kohen (following Y. Dem. VI.2.13), [and states that the sale is on the condition that "the tithes are mine for four or five years," that is permitted. [But if he told the Kohen that the sale is on the condition that "the tithes are mine] forever," that is forbidden, since no Kohen can make a Kohen (i.e., a non-Kohen does not have a permanent right to tithes). And likewise the son of a Levite that owed money to a Yisrael, he may not separate on his account [and take his portion of tithes as recompense], for no Levite can make a Levite (Y. Git. III.7.2).