Tosefta על גיטין 3:7

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).