Gemara

Shmuel taught that if Hekdesh of any value was redeemed onto a Perutah, the Hekdesh becomes Chulin.

This is b'Di'eved.

Erchin 29a: A man in Pumbedisa made his property Cherem (Hekdesh).

Rav Yehudah: Take four Zuz, redeem the property onto them, and throw them in the river, and then the property is permitted.

Suggestion: This is like Shmuel. His law is b'Di'eved when the Mikdash stands, for then there is a loss to Hekdesh;

Nowadays, without a Mikdash (in any case we get rid of the money lest someone transgress Me'ilah) this is l'Chatchilah.

Question: If so, a Perutah suffices. Why did Rav Yehudah tell him to redeem onto four Zuz?

Answer: This was to publicize the redemption.

Gitin 65a (Mishnah): One may scheme to redeem his Ma'aser Sheni without adding the additional fifth. One may tell his Amah ha'Ivriyah 'here is money. Go redeem this Ma'aser.'

Suggestion: This is for Ma'aser Sheni nowadays (after the Churban), which is only mid'Rabanan.

Objection: Nowadays we do not have Yisrael slaves, for Yovel does not apply!

Bechoros 30a: Someone suspected about Shemitah not suspected about Ma'aser, for Ma'aser must be eaten within the wall of Yerushalayim. Perhaps he considers Ma'aser to be more stringent.

Rishonim

Rambam (Hilchos Ma'aser Sheni 2:2): It is Midas Chasidus to redeem Ma'aser nowadays for its value, like was done when the Mikdash stood. Ge'onim ruled that nowadays l'Chatchilah one may redeem Shavah Maneh on Shavah Perutah. This should not be more stringent than Hekdesh, all the more so one may do so for Ma'aser. He casts the Perutah to the sea.

Ra'avad: I would be astounded if the Ge'onim said so even nowadays Peros on Peros, except for Ma'aser Sheni of Chutz la'Aretz, which they made like Demai.

Beis Yosef (YD 331 DH va'Yechalelenu): It seems that the Rambam learns from Erchin, in which Rav Yehudah told a man to cast four Zuz to the sea, to publicize the matter. The Rambam holds that only for Cherem, which would be a big loss to redeem for the full value, four Zuz suffices. For Ma'aser it is Midas Chasidus to redeem it for the full value to publicize the matter.

Rosh (Sof Hilchos Orlah): After the entire vineyard was harvested, one redeems all the grapes on one Perutah.

R. Yonah (Berachos 25a DH v'Din): We redeem all the Peros (of Revai) onto one Perutah. Bahag says that one must redeem onto four Zuz. Erchin 29a supports this. I say that one need not use four Zuz. One may use less. We asked from Shmuel's teaching that a Perutah suffices, and answered that Chutz la'Aretz is more lenient than Eretz Yisrael. In Chutz la'Aretz, it is l'Chatchilah on a Perutah. The Halachah follows Shmuel.

Beis Yosef (YD 294 DH u'Mah): R. Yonah connotes that only in Chutz la'Aretz it is l'Chatchilah to redeem on a Perutah. In Eretz Yisrael, one must redeem on the value. The Rambam and Rosh said that nowadays one may redeem on a Perutah. They did not distinguish between Eretz Yisrael and Chutz la'Aretz. I do not understand R. Yonah's support from Erchin. The Gemara distinguished between nowadays and the time of the Mikdash. It distinguished between Eretz Yisrael and Chutz la'Aretz only regarding a Cherem field, but not regarding Revai.

Bach (DH Kasav): Perhaps R. Yonah holds like the Ge'onim that we learn from Erchin to everywhere. The Shulchan Aruch brings their opinion in CM 95:4, that we do not swear on land only in Eretz Yisrael, but in Chutz la'Aretz we swear, for it is like Metaltelim. The Ra'avad says that this is only for Cherem, but R. Yonah holds like the Ge'onim.

Tosfos (Megilah 10a DH d'Kulei and Beitzah 5b DH u'Vikesh): R. Eliezer wanted to make his Kerem Revai (fourth-year produce) Hefker for the poor. How would this help them? They would need to redeem it, for there was no wall around Yerushalayim! I answer that they could profit, for one can redeem it for a Perutah, like Shmuel. Shmuel's law is b'Di'eved only when the Mikdash stands. Nowadays it is l'Chatchilah. The She'altos (Kedoshim Sof 100) says so.

Tosfos (Rosh Hashanah 31b DH u'Vikesh): We cannot bring a proof from Hekdesh to Ma'aser Sheni and Revai, for in Gitin the Gemara suggested that the Mishnah discusses Ma'aser Sheni nowadays. If one may redeem Ma'aser Sheni for a Perutah nowadays, there would be no need to scheme! However, Bahag and the She'altos say that one may redeem it for a Perutah. The Gemara in Gitin could have asked this, but it did not. (It asked a stronger question, that nowadays there is no Amah Ivriyah.

Tosfos (Gitin 65a DH v'Amah): If the Mishnah discusses Ma'aser Sheni nowadays, one need not scheme, for one may redeem it for a Perutah! According to the opinion that the Chomesh must be worth a Perutah, one may redeem it for five Perutos including the Chomesh. (Through the scheme, he may redeem it for one Perutah, since he need not add a Chomesh.)

Tosfos (Bechoros 30a DH b'Asrei): Why are people suspected about Ma'aser more than Shemitah? (Even Amei ha'Aretz) are not suspected about Terumah, because one can exempt the entire harvest with one piece of wheat. Likewise, one can redeem all the Ma'aser on one Perutah, like Shmuel taught regarding Hekdesh. The She'altos says that the same applies to Revai, which has the same law as Ma'aser Sheni. Presumably, this is l'Chatchilah, like we find in Erchin. Nowadays, it is common to redeem Revai on a Perutah!

Answer (Tosfos): Ma'aser is unlike Terumah. Sometimes one does not have a Perutah to redeem the Ma'aser. Also, sometimes one has only a little Ma'aser, and he does not want to give a whole Perutah offer it.

Poskim

Shulchan Aruch (YD 294:6): Nowadays that we cannot bring Revai to Yerushalayim (to eat them there), one can redeem them for Shaveh Perutah even if there are many Peros.

Gra (20): Tosfos (Berachos 35a DH ul'Man) says that nowadays we redeem on a Perutah, and the Rosh and Rambam say so in the name of Ge'onim. It is no more stringent than Hekdesh. I question the proof from Hekdesh, for Ona'ah does not apply to Hekdesh. Perhaps nowadays, Ma'aser Sheni is not worth anything. However, it is unlike Hekdesh. Further, the Amora'im, who were after the Churban, used to redeem it for its value, like it says in the Yerushalmi. To answer this, the Rambam said that it is Midas Chasidus to redeem it for its value, like when the Mikdash stood. This does not answer the question, for they (Amora'im) ruled in practice (that one must redeem for the value). It is difficult to say that all of them argue with Shmuel. Rather, the Ge'onim hold that Ma'aser Sheni and Revai are like Hekdesh, like R. Meir (Kidushin 54b). The Yerushalmi holds like R. Yehudah (that Ma'aser Sheni and Revai are unlike Hekdesh) against R. Meir. Chachamim say so in Pesachim 37,38 and Sukah 35. We hold like the Bavli. This is why the Terumas ha'Deshen says that there is not even Midas Chasidus to redeem it for its value. Even though in Me'ilah we say that this is only b'Di'eved, and in Bava Metzi'a and Temurah Amora'im argue about this, Tosfos says that all agree that it is only b'Di'eved. However, nowadays it is l'Chatchilah.

Gra (Likut): According to what I wrote, the Mishnah in Gitin is not difficult. It is like R. Yehudah (who holds that Ma'aser Sheni is unlike Hekdesh, therefore one must redeem it for its value). The Gemara established the Mishnah in Bechoros to be like R. Akiva, who holds like R. Yehudah. This answers also the question of Tosfos there. The reason one suspected about Shemitah is not suspected about Ma'aser is because Ma'aser must be eaten within the wall. We did not say that it is because one can redeem it for a Perutah!

Minchas Shlomo (1:71:20): The Gra says that the Rambam's source is the Yerushalmi, but the Bavli rules like R. Meir, that Ma'aser Sheni is like Hekdesh, therefore there is no Ona'ah. This is why the Terumas ha'Deshen rules that there is no Midas Chasidus to redeem it for its value. It seems that the Gra does not rule like the Terumas ha'Deshen against the Rambam. Rather, he teaches that b'Di'eved one need not be concerned for the Yerushalmi, since the Terumas ha'Deshen holds that there is not even Midas Chasidus.

Shulchan Aruch (331:133): Nowadays, if one wants to redeem Shavah Maneh of Ma'aser on a Perutah, l'Chatchilah, he does so, and casts the coin to the sea.

Beis Yosef (DH va'Yechalelenu): It seems that the Rambam learns from Erchin, in which Rav Yehudah told a man to cast four Zuz to the sea, to publicize the matter. The Rambam holds that only for one who made (all) his property Cherem, which would be a big loss to redeem for the full value, four Zuz suffices. For Ma'aser it is Midas Chasidus to redeem it for the full value to publicize the matter.

Shulchan Aruch (ibid.): Similarly, if he redeemed Shavah Maneh of Ma'aser on Shavah Perutah of other Peros, it is redeemed

Gra (201): The Mechaber holds that this is l'Chatchilah.