משנה על ויקרא 7:18

One may not buy terumah with maaser sheni money, because this reduces the number of those who can eat it. But Rabbi Shimon permits it. Rabbi Shimon said to them: If the law is lenient in the case of wellbeing offerings, though they may become unfit or a remnant or unclean, why should it not also be lenient with regard to terumah? But they said to him: The law was lenient in the case of wellbeing offerings, because they are permitted to non-priests, but should we therefore be lenient with regard to terumah, which is forbidden to non-priests?