The Mishnah (3:2) teaches that one cannot purchase terumah with maaser sheni money "because it reduces its consumption". R' Shimon however permits the purchase. We shall try to understand the debate.
The Bartenura explains the Chachamim's objection as follows. Since a tevul yom (one that has immersed in a mikveh and is waiting till nightfall to become completely tahor) and a zar (non-kohen) are forbidden from consuming terumah, using maaser sheni money to purchase terumah would restrict the number of people that can eat the maaser sheni. The Tosfot R' Akiva adds that the purchase would also restricting the number of people that can consume the terumah. An onen (a mourner prior to burial) can eat terumah but not maaser sheni. Furthermore, terumah can be eaten anywhere while maaser sheni can only be consumed in Yerushalaim.
The Bartenura however explains that R' Shimon is not bothered by this, since in general he maintains that one can do things that can "cause kodshim to become pasul".
Indeed, the Mishnah continues that R' Shimon argues that the Chachamim have no problem with using maaser sheni money to purchase a korban shelamim, even though the korbal could become pasul in several way (pigul, notar, tameh, etc). The Chachamim however respond that a korban shelamim is still permitted to everyone, unlike terumah that is only permitted to kohanim. Consequently, no conclusions can be drawn from the permit to purchase a korban shelamim, to whether one can purchase terumah. The Tifferet Yisrael add that even though parts of the korban cannot be consumed, this is no different to purchasing jugs of wine, which we learnt last week is permitted.
Interestingly, the Yerushalmi cites a beraita, that the issue is that one does not cause the maaser sheni to became pasul. R' Yonah explains that maaser sheni not normally affected by a tevul yom. If however a tevul yom touched this maaser sheni terumah, it would become pasul and one would not be able to consume it. This explaining aligns more directly with the issue of "causing kodshim to become pasul".
The Mareh Panim understands that the Mishnah and Beraita argue regarding the reason behind the Chachamim. According to our Mishnah simply restricting the population that would have been able to eat the maaser sheni is a reason for concern, while the Beraita understands that that is not enough. It must be that purchasing the terumah could result in the maaser sheni becoming pasul.
The Mareh Panim continues that another discussion in the Yerushalmi in only understood according to the Beraita's understanding. Another Beraita teaches that one cannot purchase shemittah produce with maaser sheni money. The concern is that once the time of biur arrives for the shemittah produce, if the owner did not render it hefker (ownerless) it would be forbidden, thus causing a loss to the maaser sheni. R' Yosi explains that this law would also be part of the debate between the Chachamim and R' Shimon. R' Yona however disagrees. He explains that R' Shimon allowed the purchase of terumah since kohanim are very particular and will ensure the terumah will not be come tameh. Shemittah produce is in the hands of everyone. Consequently, even R' Shimon would agree it should not be purchased with maaser sheni money. The Mareh Panim explains that one can only understand this discussion if the issue is making the maaser sheni terumah invalid. In that case, whether the kohanim are fastidious can make a difference. If however the concern is simply reducing the audience that can eat maaser sheni terumah, it is more difficult to understand this discussion.1
The Mishnah Rishona however explains that the concern in our Mishnah, is that with a restricted population that can eat the maaser sheni terumah, if there are not enough kohanim to eat the food, it will go to waste. The being the case, the restricted population is what might cause the maaser sheni to be wasted, thereby aligning the Mishnah and Beraita.
1 From this discussion it would appear that R' Shimon is concerned about bringing kodshim to become pasul in some cases. See the Mareh Panim who explains why this is still not the case.
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