Maaser Sheni, Chapter Three, Mishnah Two
In this mishnah we learn that one cannot use second tithe money to buy terumah.
1) One may not buy terumah with maaser sheni money, because this reduces the number of those who can eat it.
a) But Rabbi Shimon permits it.
2) 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?
3) 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?
Section one: 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.
Section two: 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.
Section three: 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.