פירוש על מעשר שני 3:2

מפני שהוא ממעט באכילתו – for heave-offering is forbidden to one who has bathed but must wait until sunset to be perfectly clean and to foreigners (i.e., non-Kohanim).

Introduction In this mishnah we learn that one cannot use second tithe money to buy terumah.

ורבי שמעון מתיר – who holds that it is permitted to bring holy things to the house of defilement/blemish.

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. Here we learn that there is a debate with regard to buying terumah with maaser sheni money. This probably refers to a priest who has maaser sheni money and wants to use it in Jerusalem to buy terumah, which is cheaper than hullin, non-sacred produce. The sages forbid this because it reduces the number of people who can eat the maaser sheni. Produce bought with maaser sheni can be eaten by anyone, but terumah can only be eaten by priests. Rabbi Shimon permits.

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? Rabbi Shimon argues that just as it is permitted to use maaser sheni money to buy wellbeing offerings, even though these sacrifices might eventually be forbidden for anyone to eat, so too we should allow one to buy terumah with maaser sheni money. In other words, even though there is a reduction in the general edibility of the food bought, this does not cause the purchase to be prohibited.

מה אם היקל בזבחי שלמים – that one purchases peace offerings with the monies of Second Tithe, and even though he lessens his eating [of them] for they are able to become rejected in consequence of an improper intention on the part of the officiating priest and the remnant [which are portions of sacrifices left over beyond the legal time and bound to be burnt] which become impure, shall we not be lenient concerning the heave-offering to purchase it with the monies of Second Tithe which do not have the laws of improper intentions on the part of the officiating priest and the remnants of sacrificial portions left over beyond the legal time nor the law of impurity, for not all the things that make that which is holy impure make the heave-offering impure. And the Halakha is not according to Rabbi Shimon.

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? The other rabbis argue that the two cases are not analogous. The wellbeing offering can be eaten by anyone, and although it might become prohibited for anyone to eat, it also might never become prohibited. In contrast, terumah is always prohibited to non-priests, so there is a definite reduction in who can eat it. Therefore, one should not buy terumah with maaser sheni money.