Mishnà su Levitico 7:78

Meisah: Bet Shammai exempts [from hallah], And Bet Hillel makes liable [for hallah]. Halita: Bet Shammai makes liable, And Bet Hillel exempts. The loaves of the thanksgiving sacrifice and the wafers of a nazirite: if one made them for oneself, they are exempt [from hallah]. [If one made them] to sell in the market, they are subject [to hallah].

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?