Gemara

(R. Yochanan): If one is Matfis Ma'aser Sheni for Shelamim, it takes effect;

(R. Elazar): It does not take effect.

According to R. Yehudah, who says that one owns his Ma'aser Sheni (it is Chulin, just there are restrictions), all agree that Kedushas Shelamim is strong enough to be Chal. They argue according to R. Meir, who says that Ma'aser Sheni is Kodesh. (One does not own it, he just has permission to eat it);

Clearly, R. Elazar holds like R. Meir;

Even R. Yochanan's law can be like R. Meir;

Ma'aser is called Shelamim (it says "v'Achalta Sham (Shelamim)." A Gezerah Shavah teaches that this refers to an animal bought with Ma'aser.)

Kidushin 54b (Mishnah): If one did Meshichah on Ma'aser Sheni (picked it up or brought it to his domain to buy it) when it was worth one Sela, but he did not redeem it until it was worth two Sela'im, he pays one Sela and profits a Sela. The Ma'aser Sheni is his.

This is unlike R. Meir. He holds that Ma'aser is Mamon Gavoha (Hash-m's property) - "v'Nasan ha'Kesef" "v'Kam Lo" (he will give the money, and it will be to him." Rather, it is like R. Yehudah.

Seifa of the Mishnah (Ma'aser Sheni 4:6): If one was Moshech Ma'aser for two, and before he redeemed the price became one, he gives a Sela of Chulin and the Sela of Ma'aser is his.

Bechoros 51a (Mishnah): One may not redeem with slaves, documents, land or Hekdesh.

Rishonim

Rambam (Hilchos Ma'aser Sheni 8:7): If one bought Peros for a Sela of Ma'aser Sheni and was Moshech, and before he paid for them the price increased to two, he separates only a Sela, for it says "v'Nasan ha'Kesef v'Kam Lo". One acquires through giving money, and Ma'aser profits.

Rebuttal (Ra'avad): This must be a mistake. He did not give money. He acquired through Meshichah, for a commoner should not have greater rights than Hekdesh! Another mistake is that if he bought Chulin Peros to give Ma'aser money for them, what did the Ma'aser do? He was Moshech Chulin Peros. Is he an Apotropos (overseer) of Ma'aser, so that his Yadhand (power to acquire) is like that of Ma'aser?! The Mishnah discusses one who was Moshech Ma'aser to buy it for a Sela, and and before he paid, the price increased to two. This connotes that the Peros were Ma'aser. He acquired through Meshichah, but not because a commoner is no greater than Hekdesh. When it is acquired for a Sela, this is not is Ko'ach (strength) of Hekdesh. It is its weakness, that it is Mekadesh only a Sela for its redemption! The Yerushalmi (Ma'aser Sheni 4:6) said that the Mishnah is unlike R. Shimon (ben Gamliel), who says that Meshichah of Ma'aser Sheni is redemption. This implies that the Mishnah holds that Meshichah does not acquire! The seller profits the Sela, and the Ma'aser Sheni is his. He may take a Sela of the Ma'aser (Peros). I am uneasy with this. Rather, the entire Mishnah is letter of the law. Ma'aser is redeemed only when the money is given, but we judge the buyer and seller based on when Meshichah was. If the price was a Sela at the time of Meshichah, and it rose to two, he pays a Sela and the buyer profits a Sela, and he owns them (the extra Sela of Peros) like Ma'aser, and he eats then in Yerushalayim or redeems them. If the price was two at the time of Meshichah, and it decreased to one, he redeems them for a Sela of Chulin, and pays another Sela to the seller, like the price at the time of Meshichah. He may even give the other Sela from Ma'aser, for it is a mere debt. Even though one may not pay a debt with Ma'aser, here is different, for the debt was due to Ma'aser (Yerushalmi, Reish Ma'aser Sheni). If the seller is an Am ha'Aretz, one may give him a Sela of Ma'aser Sheni of Demai, but not of Vadai Tevel. The Gemara says that he does not add a Chomesh, i.e. if and when he will redeem the extra Peros. It is considered redeeming the seller's Ma'aser, which he acquired through his payment. This is unlike R. Shimon, who says that redemption is based on the time of Meshichah.

Radvaz: The Rambam calls the Peros Ma'aser because they will be Ma'aser. The sale is called redemption, because the money becomes Chulin. Letter of the law, the buyer should profit, and not Ma'aser, for he acquired the Peros through Meshichah and the money is not yet Chulin. It is as if he consumed two Sela'im of Ma'aser, for now it is worth two. V'Nasan ha'Kesef v'Kam Lo teaches otherwise. Granted, he acquired through Meshichah, lest a commoner have greater Ko'ach than Hekdesh, i.e. regarding the seller the sale is finished. Since he will pay with Ma'aser, for the buyer it is not finished until he pays, and the Peros become Ma'aser. However, 'he profits a Sela, and the Ma'aser is his' connotes that the buyer profits and the Ma'aser is considered like two. Perush ha'Mishnayos says so. In the Mishnah Torah, he retracted. It is reasonable that Ma'aser profit; one should not do business with Ma'aser, which is Mamon Gavoha. The Mishnah means that the buyer keeps the extra Sela, and it is Ma'aser.

Rosh (Ma'aser Sheni 4:6): One may not sell Ma'aser to take it to Yerushalayim. Our Mishnah discusses buying to transfer the Kedushah to the money.

Kesef Mishneh: The Yerushalmi says that our Mishnah is unlike R. Shimon, who says that Meshichah of Ma'aser is always the redemption. The Rambam explains our Mishnah to say that Meshichah acquires for the person, but redemption is when he pays. Therefore, in the Reisha all the Peros become Ma'aser, and in the Seifa he may pay only one Sela from Ma'aser. R. Shimon argues only in the Seifa; he holds that one pays both Sela'im from Ma'aser. This is why he said 'always.' However, the Ra'avad's second objection is strong. How does v'Nasan ha'Kesef v'Kam Lo teach that he pays based on the time of Meshichah?! Perhaps the Rambam explains that when he will pay, it (the increased value at the time) will be for him. In Kidushin, we establish the Mishnah like R. Yehudah, who says that Ma'aser is Mamon Hedyot (a person's property). We rule like R. Meir, that it is mmg! Even so, the Rambam brought the Mishnah, but changed the Perush to be stringent about Ma'aser, for it is Mamon Gavoha.

R. Chayim ha'Levi on the Rambam: The Rambam holds that the Peros were Chulin. The Ra'avad asks, if so v'Nasan ha'Kesef v'Kam Lo does not apply to Meshichah! I answer that Meshichah obligates the buyer to pay for what he took. Kinyan Kesef acquires the item for which the money is given. Shavah Kesef (something that can be sold) acquires like Kesef. Therefore, Meshichah of the Peros acquires the Kesef Ma'aser (like Shavah Kesef acquires Hekdesh). Hekdesh can be redeemed on Metaltelim (Bechoros 51a) because Shavah Kesef is like Kesef. The Mishnah discusses a purchase (Kinyan) of Peros that also entails Chilul. The Torah decreed that Chilul depends on giving the money. Kinyan depends on Meshichah. If Ma'aser is Mamon Gavoha, the Peros acquire (and receive the Kedushah of) the money (once Meshichah is done, even before the money was given), for this is like Shavah Kesef acquiring Hekdesh. If so, Ma'aser profits (all the Peros become Ma'aser due to one Sela). The Mishnah says that the buyer profits. This is why the Gemara said that the Mishnah holds that Ma'aser is Mamon Hedyot. The law of Hekdesh does not apply, so there is no Chilul until the money is given, and then the Peros were worth two, so the buyer can redeem two Sela'im of Ma'aser on the Peros. The Rambam rules the first way, for we hold that Ma'aser is Mamon Gavoha.

Rambam (8): If one was Moshech Peros worth two, and before he paid for them the price decreased to one, he separates only a Sela of Ma'aser coins, and gives it and another Sela of Chulin to the seller.

R. Chayim ha'Levi: At the time of Meshichah, it is not yet determined on which coin of Ma'aser they will be redeemed. This depends on Breirah (something taking effect now based on future events). We do not rely on Breirah for Torah laws! Also, why does the Rambam say that when the price dropped, he redeems only one Sela of Ma'aser on them? Since the Chilul is at the time of Meshichah, he should redeem on them their value at that time (two Sela'im)! I answer that Chilul occurs through Kinyan Kesef (or Shavah Kesef), at the time of Meshichah, even though the coin is not yet determined. This is like a Kinyan now to take effect later. Regarding the Peros, which are Chulin, it depends on their value at the time of Meshichah. Regarding the Kesef, it depends on when it is given and it becomes Chulin. When the Peros rose in value, since Ma'aser cannot be Tofes (give Kedushah to) more than its own value, the question is whether all the Peros receive Kedushah from one Sela of Ma'aser. Peros depend on Meshichah, which was when they were worth only one (so they are all Nitfas on one Sela). When the Peros decreased in value, the question is whether the Kesef can be redeemed on less than its value. Kesef depends on the time the Kesef was given, and then the Peros were worth only one (so only one Sela becomes Chulin). This is unlike the Mishnah, which holds that Ma'aser is Mamon Hedyot, so the Chilul is always based on when the money is given.

R. Chayim ha'Levi: The Ra'avad asked that the buyer is not a treasurer of Hekdesh. I.e. he is a commoner buying from a commoner. The Chilul is separate matter. V'Nasan ha'Kesef v'Kam Lo does not apply! The Rambam holds that Meshichah is also Kinyan (Shavah) Kesef, i.e. it is a Kinyan of Ma'aser Sheni. v'Nasan ha'Kesef v'Kam Lo teaches that the Kinyan itself is Chilul, even if the action of Chilul did not yet occur and nothing entered in place of it.

Ohr Some'ach (7): The text of the Rambam should say 'if he was Moshech the Peros and the price increased to two, he separates only a Sela, and Ma'aser profits. Halachah 8 should say 'if he was Moshech Peros worth two, and before he paid the price decreased to one, he separates only a Sela of Ma'aser, for it says v'Nasan ha'Kesef v'Kam Lo. He adds another Sela of Chulin and gives it to the seller, for Chilul is only through Kesef.