Gemara

(Beraisa): If Kohanim, Leviyim or Aniyim (poor people) help in the pen, granary or slaughterhouse (in order to receive Matanos), we do not give to them Terumah or Ma'aser.

If one gave to them, he profaned the gifts. It says "Shichatem Bris ha'Levi," and "v'Es Kodshei Venei Yisrael Lo Sechalelu v'Lo Samusu."

The second verse teaches that he is Chayav Misah (b'Yedei Shamayim) for this.

51a (Mishnah): If Reuven wrote a document to a Kohen saying 'I owe you five Shekalim', his son is not redeemed. He must pay;

Therefore, if the Kohen wants, he may return the money.

51b (Beraisa): If the Kohen took the money and returned it, the Pidyon is valid;

R. Tarfon used to do so.

R. Chanina used to take the money and return it. Once, he saw a man lingering around after giving the money (to get it back).

R. Chanina: I see that you did not intend to give it. You acted wrongly. Your son is not redeemed!

Kidushin 6b (Rava): If one said 'here is money Al Menas (on condition that) you return it to me', it cannot redeem a Bechor. One fulfills the Mitzvah of giving Terumah even if he stipulates that the Kohen return it.

However, this is forbidden, for it is like a Kohen who helps in the granary in exchange for receiving Terumah.

Objection #1: If Rava holds that a gift Al Menas to return it is considered a gift, it should work in both cases! If it is not considered a gift, it should not help for Terumah, either!

Objection #2: Regarding Esrog, Rava considers it to be a gift!

(Rav Ashi): Rather, such a gift works in both cases.

(Rav Huna Mar brei d'Rav Nechemyah): Rava actually taught like you said.

Rishonim

Rambam (Hilchos Bikurim 11:8): If the Kohen wants, he may return the money of Pidyon ha'Ben. One may not give with intent that the Kohen will return it. If he did, and the Kohen returned it, his son is not redeemed until he resolves in his heart to give an absolute gift. If afterwards the Kohen wants to return, he may. Similarly, if he stipulated and gave 'Al Menas you will return the money', his son is redeemed..

Rosh (8:10): R. Chanina told the man that since he did not intend to give it, even if R. Chanina would not return the money, his son is not redeemed. Why is this unlike a gift Al Menas to return it? That works for Pidyon, and all the more so here, that he did not stipulate! We can say that R. Chanina wanted to take only a proper gift, lest he "corrupt the covenant of the Levi." However, since he and R. Tarfon always used to return, Shichatem Bris ha'Levi applies to them! People would give to them, and not to other Kohanim! We can say that since the giver did not know at the time that they would return, Shichatem Bris ha'Levi does not apply. Later, they returned it for a pure gift. Also, perhaps they did not return all the time, rather, only to Aniyim.

Gra (YD 305:14): (Also) Tosfos (51b DH Hilkach) asked why this is different than a gift Al Menas to return it. There, he gives a proper gift. The Tanai is a separate matter. If he did not stipulate, and gave and said '... and you will return it', this is nothing. This is why one cannot give a Lulav to a child on Yom Tov (and be Yotzei with it afterwards, because he cannot stipulate that the child return it).

Rashba (1:198): If a Yisrael gives five Sela'im or their value to a Kohen, the Kohen may return from them as much as he wants, be it some or all. After he acquired, (if he gives) it is a gift of his property. One may stipulate 'here are five Sela'im for Pidyon of my son, Al Menas you will return them to me', for saying Al Menas is like saying 'from now' (Kidushin 6b). This is only if he said Al Menas, but if he said 'here are five Sela'im for Pidyon of my son, and you will return them to me', without Al Menas, his son is not redeemed. Even if he did not say anything, just he intended that the Kohen return all or part of it, and if not, he would not give to him, even if the Kohen did not return anything, his son is not redeemed, like it says in Bechoros 51b.

Rashi (51a DH Lefichach): Since Reuven must pay the five Sela'im, the only solution (to enable the Yisrael to keep his money), is that the Kohen takes it, and then he may return the money.

Poskim

Shulchan Aruch (YD 305:8): If a Kohen wanted to return Pidyon ha'Ben, he may. However, one may not give to a Kohen Al Menas that he return it. If he did so and he returned it (Rema - and initially the Kohen did not accept it Al Menas to return it), his son is not redeemed until he intends for a total gift. The Kohen should not regularly return to everyone, lest he cause a loss to other Kohanim, for Yisre'elim will give only to him.

Gra (12): Tosfos forbids returning it all the time. Rashi and the Rambam hold that the Isur applies only to Terumah.

Shulchan Aruch (ibid.): One may return to Aniyim all the time. All the more so, one may not accept the Pidyon Al Menas to return it. If he transgressed and accepted it, and stipulated that he gives it Al Menas to return it, his son is redeemed, for a gift Al Menas to return it is a gift.

Rema: This is only if he said 'Al Menas to return it', but if he said 'here are five Sela'im and you will return them to me', this is not a gift and his son is not redeemed.

Prishah (8): The Tur holds that it helps to say at the time he gives the money (that it is Al Menas to return it). This is unlike Rashi, who says that there is no solution.

Drishah (2): The Rema is like the Rashba. He says that saying 'Al Menas' is like 'from now'. I.e. retroactively it is revealed that it was a total gift when he gave it (CM241:9). Regarding Esrog, the Tur (658) says that a gift Al Menas to return it works, as long as he gives an absolute gift Al Menas to return a gift. If he gave it only for the receiver to be Yotzei with it, it does not help. Seemingly, we could say similarly here. However, the Rema connotes that his initial words 'here is a gift' are the same in both cases. The Rashba says that it all depends on 'Al Menas'. I.e. saying 'Al Menas' shows that he intends for a total gift from now, but he stipulates that he return it. Seemingly, it depends on whether he mentioned giving and a gift. Even though he stipulated 'Al Menas that you return it', since he also said 'gift', he shows that he gives a total gift. When he says 'here are five Sela'im and you will return them', he did not mention a gift at all. The Rambam and Shulchan Aruch connote that it depends on saying 'Al Menas'. The Shulchan Aruch writes like the Rambam, but in the middle he inserted the words of the Tur. Also, the Shulchan Aruch said 'he may not give Al Menas that the Kohen return', but the Rambam said with intent that the Kohen return. The Shulchan Aruch contradicts himself, for at the end he permits Al Menas. It seems that the Rema came to resolve this. If the Kohen's intent from the beginning was not Al Menas to return, his son is not redeemed. In the Reisha, the Shulchan Aruch does not mean that he truly said Al Menas, for if so, surely the Kohen intended to return it, and his son is redeemed. Rather, if we see that the giver did not intend for a total gift, rather, that the Kohen return it, and the Kohen took it Al Menas, his son is not redeemed, like the Rambam says. The Rema says 'the Kohen did not intend from the beginning to receive it Al Menas', i.e. the giver did not verbalize his intent, or even if he said it but it seems that the Kohen did not accept with this intent. This shows that it depends on whether he said Al Menas. The first comment of the Rema means that the Kohen intended, but it was not verbalized. Even if he specified a gift, if he did not mention Al Menas, it does not work.

Taz (6): We forbid giving with an unspoken intent to get back the money. The Seifa teaches that if he speaks it out, it helps. When he does not say it, if the Kohen intends not to return it, they have different intents at the time it is given, so if he returns it later, this is not a Pidyon. If he does not return it, his son is redeemed when the Kohen keeps it, for then they have the same intent. If the Kohen intended to return it from the beginning, they have the same intent, so his son is redeemed even if he returns it later. This is like one who stipulated 'Al Menas that you will return it to me.' This is a common custom. The father leaves a big sum, and he and the Kohen intend that the Kohen will return it. Sometimes he returns everything, and sometimes he keeps a small amount. This explains also the Rema. The Drishah says that it depends on whether the Kohen intended to return like Al Menas, or in another way. This is wrong. Whenever they had the same intent, his son is redeemed.

Taz (7): When he said 'here are five Sela'im, and you will return them to me', his latter words retract and nullify his initial words (a total gift). Rashi (Gitin 72a) explains like this regarding one who said 'from today and after death.' With Al Menas, there is no retraction. He explains his words.

Shach (6): When he said Al Menas, at one point he intended for a total gift, so he was Yotzei, like the Shulchan Aruch says at the end. In the Reisha, he never intended for a gift, just that the money be returned. Darchei Moshe (6) brings like this from the Mordechai (Kidushin 481, citing Avi ha'Ezri), and the Rashba and Rambam say so. The Levush and Drishah explain the Rema like this. I disagree. Rather, the Rema explains the Mechaber as follows. In the Reisha, since he did not speak it out, the Kohen did not accept Al Menas. The Bach explains Tosfos and the Rosh like this, and we can explain the Rambam like this. Perhaps Tosfos and the Rosh mean differently. They say that R. Chanina wanted a total gift, i.e. in order that the giver will give a total gift, but b'Di'eved his son is redeemed in any case. R. Yerucham understood them like this. Surely, the Rema and Mechaber hold like I explained.

Shach (7): The Rosh holds like the version in Rashi that his son is not redeemed even if R. Chanina would not return the money. This is unlike the Rambam. This shows that the Rosh holds like the Bach said.

Shach (8): When he said Al Menas, he once intended for a total gift. If he said 'here are five Sela'im...' without Al Menas, even if he did not say anything (about returning them), but he intended that the Kohen return all or part of them, and if not, he would not give, even if the Kohen does not return, his son is not redeemed.

Chak Yakov (OC 448:17, cited by R. Akiva Eiger): It is a stringency of Chametz that a gift (to a Nochri) Al Menas to return it does not help. The Magen Avraham says that all the more so, letter of the law 'I give to you, and you will return to me' should help, just we are stringent about Chametz. This is astounding. In YD 305, based on the Rashba, we say that 'and return them to me' is not a gift at all!

Pischei Teshuvah (12): She'elas Ya'avetz (1:155) says that nowadays that the lineage of Kohanim is doubtful, the law changed. We are stringent to treat them like Kohanim in every way, and we redeem Bechoros through them, for there is no alternative, but we cannot force one to pay them due to Safek. Perhaps letter of the law they must return it. At least the Kohen himself should be stringent to refrain from Safek theft. Nowadays it is proper to redeem Al Menas to return it. A Yisrael should redeem his son with as many Kohanim as possible. Perhaps he will find one who has definite lineage. Likewise, the son of a Bas Kohen or Bas Levi should be redeemed due to Safek. The Chasam Sofer (Sof Teshuvah 291) disagrees with the Ya'avetz.

Pischei Teshuvah (14): Teshuvah me'Ahavah (1:55) says that a gift on condition to return the same matter is a gift, but a gift on condition to return other coins is a loan, so his son is not redeemed.