Gemara

(Rava): Levi holds that the seller's Keli is used for Chalipin. With the pleasure he gets that the buyer accepted his Keli, he resolves to give the land.

71b (Beraisa): If Reuven wanted to return what he borrowed from a Nochri (Tony) on Ribis, and, and Shimon said 'give the money to me, and I will pay you (later, with additional Ribis) like you must pay to Tony', this is forbidden. If Tony was there and he authorized Reuven to give to Shimon, this is permitted.

(Rav Papa): The case is, Reuven returned the money to Tony, who gave it to Shimon. One might have thought that Tony acts on Reuven's behalf when he gives to Shimon (and mid'Rabanan, we consider Tony to be his Shali'ach and this is forbidden). The Mishnah teaches that this is not so.

Kidushin 7a (Rava): If Leah told Reuven 'give money to Ploni, and I will be Mekudeshes to you', we learn from an Arev (guarantor) that this works. An Arev obligates himself, even though he does not benefit. Likewise, Leah can be Makneh (transfer ownership of) herself to Reuven without receiving the money.

Rishonim

Rif (42a): Rav Papa permits only if Reuven returned the money to Tony, who gave it to Shimon. If Reuven himself gave the money to Shimon, even in front of Tony and on his command, it is forbidden, for a Nochri has no Shlichus.

Rosh (5:53): Since Tony did not exempt Reuven, Shimon pays for Reuven the Ribis he needed to pay. Therefore, it is forbidden.

Rosh (Teshuvah 108:13): The Rif explains that there is no Heter if Reuven gives to Shimon, even if Shimon would pay Tony. If Moshe lends to David on condition that David pay Ribis to a Nochri, it is Ribis as if David paid to Moshe, even if Moshe did not owe Ribis to the Nochri. It is as if David gives to Moshe, like we say about Kidushin. The Gemara discussed paying Reuven to teach the Heter when Tony took back the money. If Reuven gave directly to Shimon, it is Ribis Ketzuzah and must be returned. Since Reuven has Achrayus (responsibility), it is as if he lent his money to Shimon. The Gemara did not give a Heter if Shimon wrote a document obligating himself to Tony; it seems that it does not help. The Yerushalmi says that there is no Heter when Reuven has Achrayus.

Rambam (Hilchos Malveh 5:3): If Reuven borrowed from Tony on Ribis, and Shimon said 'give to me the money, and I will pay you like you must pay to Tony', this is Ribis Ketzuzah. It is permitted only if Tony took back his money and gave it to Shimon.

Question (Tosfos Bava Metzia 71b DH Motz'o): Why does the Beraisa forbid Shimon to pay to Reuven like Reuven must pay to Tony? Even if Shimon will pay Tony directly, it is permitted only if the Nochri authorized it! One may not lend to a Yisrael on condition that the Yisrael pay more to someone else! This is forbidden even if Reuven does not owe the one who will receive! Giving to a recipient is like giving to the lender, due to the law of Arev!

Answer (Tosfos): Indeed, it is forbidden even if Shimon will pay Tony directly. The Beraisa teaches the Chidush that even when he pays Reuven, it is permitted if the Nochri authorized it.

Hagahos Ashri (5:47): Levi may not say to David 'give three Zuz to Ploni, and I will lend to you.' Giving to Ploni on Levi's command is like giving to Levi, due to the law of an Arev (guarantor).

Mordechai (327,328): If Reuven borrowed from the Gabai Tzedakah on Ribis Ketzuzah (and could not evade paying it) and gave securities, and later Shimon asked to take the principal and he will pay all that Reuven needed to, this is Ribis Ketzutzah. If Shimon pays more than he received, it is returned. Even if Reuven says 'I will lend to you, if you will pay one Zuz to Levi', this is Ribis Ketzutzah. This is unlike Moshe paying David so he will lend to Ploni or tell Ploni to lend to Moshe. Here, Shimon pays Reuven's debt, more than he received from Reuven. Reuven may not lend 100 to Shimon on condition that Shimon pay 20 to Hekdesh and return the 100 to Reuven. What he pays due to the lender's command is as if he returns to the lender, like we say regarding Kidushin.

Tosfos (57b DH Lesapek): If Reuven lent 100 to Shimon on condition that Shimon pay 120 to Hekdesh, this is Ribis mid'Oraisa, due to the law of Arev.

Poskim

Shulchan Aruch (YD 160:14): One may not say 'I will lend to you 100 (Zuz), on condition that you give one to Ploni (Rema - or to Hekdesh), even if Ploni is a Nochri, even though he does not owe to him. This is Ribis Ketzutzah (he stipulated how much to give in order that he will lend to him).

Beis Yosef (DH Kosav): The Mordechai forbids lending 100 to Hekdesh on condition to get back 120, and all the more so to lend to a person on condition that he will pay back 100 and give 20 to Hekdesh.

Rema: It is the same if the lender said so, or the borrower said so and the lender lends for this reason.

Shach (19): The Mordechai says that it must be returned because the lender stipulated.

Mishneh l'Melech (Hilchos Malveh 5:14 DH v'Im): All the Poskim say so. Why did the Rosh (5:53) forbid 'because Reuven owed the Nochri'? In any case it is forbidden due to the law of an Arev! Perhaps this is only when the lender owed Ploni or asked to give to Ploni, but we permit if the borrower suggested giving to Ploni. Why should we suspect that the lender considers this as if he himself received?! Alternatively, in the Rosh's case the Nochri lends only on Ribis. If Shimon were not paying Reuven's debt, we would say that he pays only to the Nochri. If so, in a normal case a borrower may not say that he will give to Ploni if Reuven lends to him. If a man asked to be Mekadesh a woman, and she told him to throw it to the sea, Rishonim argue about this (brought in Tur EH 30). It is not clear if they similarly argue about Ribis in such a case.

Ritva (69b Sof DH Omar, citing the Ramban): Even if one told Levi 'throw a Zuz to the sea, and I wil lend to you', this is Ribis Ketzuzah, for it is as if the money came to Levi and he threw it.

Yad Avraham (YD 166:2): The Gemara (60b) could not find Neshech (the borrower loses) without Tarbis (the lender gains). Here, it is as if the lender received the money, due to the law of Arev, so there is Tarbis.

Note: I do not understand why this should be forbidden more than a standard loan with a document, in which the borrower pays the scribe.

Machaneh Efrayim (Hilchos Malveh 11 DH Ivra): The Rambam holds that Kidushin works through Tovas Hana'ah (it is worth money to her for Ploni to receive due to her). If Levi told David 'give a Dinar to Ploni, and I will lend to you' he would obligate Levi to return only his Tovas Hana'ah. Tosfos says that Kidushin works because it is as if she received the money; here, he would obligate Levi to return the Dinar. However, perhaps Tosfos says so only about Kidushin, in which he must give to her. Here, the loan does not depend on giving to Ploni. It is a mere gift. Levi 'receives' only Tovas Hana'ah. The Ritva explains Kidushin like the Rambam. Why does he say here that it is as if the money came to Levi? If Levi told David 'give a Dinar to Ploni for a gift, and I will lend to you', all agree that it is not Ribis Ketzuzah.

Shulchan Aruch (169:1): If Reuven borrowed from Tony on Ribis, and Shimon said 'give to me the money, and I will pay you like you must pay to Tony', it is forbidden. This is even if Shimon wrote a document obligating himself to Tony, gave securities and gave the Ribis to Tony.

Birkei Yosef: The Tur connotes that if Reuven had no Achrayus, e.g. he borrowed with a security and Tony relies solely on the security, Shimon may take the money and pay later to Tony like Reuven needed to. The Shulchan Aruch (Sa'if 17) says so.

Rema: Everything he paid to Tony, Reuven must pay back to Shimon, for it is Ribis Ketzuzah.

Chavos Ya'ir (190): If Shimon was unable to pay his loan and Reuven demanded an Arev, and Shimon promised to give to Levi a Dinar each month if he will agree to be an Arev, this is permitted. However, if Reuven knew that he cannot find an Arev other than Levi, and he intended that Levi will profit, this is like 'give a Dinar to Ploni and I will lend to you', which is forbidden. There is no difference whether this was at the beginning of the loan or not, just the Rambam holds that if there was no stipulation at the beginning, it is only Avak Ribis. However, if the time for payment came and Levi paid Reuven and now Levi asks Shimon to pay him, he may no longer accept money for giving Shimon time to pay, for this is Ribis Ketzuzah according to Rashi, and Avak Ribis according to the Rambam. This is when it was initially a loan. If a debt did not come from a loan, rather it was wages, or he owed due to Ma'amad Sheloshtam or Shibud d'R. Noson, or one collected documents for his loan and now tries to collect them, or a creditor took a buyer's property and he demands compensation from the seller, Ploni may give money to appease the one to whom he owes to give to him time to pay.