Gemara

69b (Rava): Reuven may tell Shimon 'take four Zuz and lend money to Levi.' The Torah forbids only Ribis paid from the borrower to the lender.

(Rava): Reuven may tell Shimon 'take four Zuz, and ask Levi to lend to me.'

The money is wages for Shimon to speak on his behalf.

Rishonim

Mordechai (338): In a Teshuvah, Rashi permits David to tell Levi 'lend to me Moshe's money, with Ribis.' The loan and Ribis is sent through Levi. The Torah forbids only Ribis that the borrower gives directly to the lender. Here, Levi does not transgress, for the money is not his. The Meshale'ach does not transgress, for Ein Shali'ach l'Devar Aveirah. One should not publicize this.

Poskim

Rema (YD 160:16): Some permit David to tell Levi 'lend to me Moshe's money, with Ribis', for the borrower does not give directly to the lender. The loan and Ribis is sent through Levi. The Shali'ach does not transgress, for the money is not his. David and Moshe do not transgress, for Ein Shali'ach l'Devar Aveirah. One should not publicize this in front of Amei ha'Aretz. Some question this Heter and are stringent to forbid it; one may rely on it when needed. If the borrower took the money straight from the lender, Ribis is forbidden even through a Shali'ach!

Rebuttal #1 (Beis Yosef DH v'Chosav): No Posek holds like Rashi. All say that it is Ribis. The Tur says that if one was paid to ask Ploni to lend (Sechar Amirah), Ploni may not take the money from him. Also, we forbid lending with Ribis through a Nochri. One may not rely on this at all. It seems that Rashi never permitted this. An errant Talmid wrote this and attributed it to Rashi.

Defense (Darchei Moshe 6,7 and 169:3): Even Rashi forbids Ribis through a Shali'ach when Shimon borrowed directly from Ploni. In this case, the Tur forbids only to take Sechar Amirah. Even Rashi forbids Ribis through a Nochri. He is stringent to say that a Nochri can be a Shali'ach. Yesh Shali'ach (l'Devar Aveirah), for Ribis is permitted to Nochrim. In any case, l'Chatchilah it is good to be stringent not to rely on Rashi's Heter.

Chidushei Anshei Shem (60, on Mordechai 325): Here, the Mordechai forbids taking Sechar Amirah. This is when Almoni directly lent to the borrower.

Rebuttal #2 (Levush 13): Rashi holds that 'Ein Shali'ach' voids the Shelichus. Why was the Rema lenient? The Beis Yosef said that all Poskim disagree! In every case it is forbidden mid'Oraisa. We say Ein Shali'ach, for the Shali'ach must heed Hash-m. He sins, for the Aveirah was done, like if one ordered a Yisrael to eat pork. If the Shali'ach was Mezid, he causes transgression of taking and paying Ribis. If he was unaware of the Ribis, the Meshale'ach transgresses Lifnei Iver (making another transgress).

Defense (Mishneh l'Melech Hilchos Malveh 5:14 DH ha'Oleh): The borrower didn't pay the lender, for Ein Shali'ach. No Isur was done, unlike 'go eat pork!'

Levush: If the Torah permitted Ribis via a Shali'ach, it would be a mockery in the eyes of the masses. It would be better not to forbid at all, than to forbid it and permit it in a way that anyone may breach it. The Isur is not only to pay Ribis directly to the lender. It is so that "your brother will live with you". The lender may not profit at the borrower's expense, even if he pays via Shali'ach! People would always lend this way. It is easy to find a Shali'ach in Yisrael!

Note: A rich lender might be ashamed to ask a Shali'ach to lend to a poor widow with Ribis! The Torah gives many loopholes. One may redeem much Hekdesh on a Perutah. Giving any share of one's animal to a Nochri exempts from Bechorah and Ma'aser. Taking Peros into the house through the roof exempts from Ma'aser. Exempting borrowers from paying until after Shemitah avoids cancellation of loans...!

Levush: RashI say not to publicize this to Amei ha'Aretz. We need not cover up what the Torah permits! The Torah was given openly to all! One who lacks money (to lend) may pay his own money to a lender to give a loan and help the borrower. Carefree people and Amei ha'Aretz read the Rema and permit l'Chatchilah, and violate even Rashi's opinion. Ribis became a joke in their eyes. Surely the Meshale'ach transgresses all the Lavim via the Shali'ach.

Defense (Shach 22): The Darchei Moshe answered the Beis Yosef's questions. We find that Chachamim commanded to conceal Heterim from Amei ha'Aretz, e.g. that one fulfills Talmud Torah through saying Kri'as Shema. The Maharik, Binyamin Ze'ev and Bach defend the Rema's ruling.

Rebuttal #3 (Taz 11): It is no different if David lends directly to Moshe. If the Shali'ach is in place of David, when Moshe gives to him, this is like paying Ribis to the lender. When the Shali'ach pays David, he is like a borrower paying Ribis to the lender! A Yisrael may not be a Nochri's Shali'ach to lend to a Yisrael, for Ein Shelichus l'Nochri. Here also, Ein Shali'ach l'Devar Aveirah! He is like an involved party dealing with the borrower and lender.

Defense (Nekudas ha'Kesef): Borrowing through a Shali'ach is not like borrowing directly, for Ein Shali'ach. When he borrowed directly, what he pays extra is Ribis, and Ein Shali'ach does not permit it.

Taz (ibid.): Also, if one may pay Ribis through a Shali'ach, what Aveirah is it to borrow straight from the lender? One may say 'lend to me with Ribis', as long as he does not pay Ribis.

Question (Gilyon Maharsha): One may not say so, due to "Lo Sesimun"!

Taz (ibid.): Even if you will say that one may not say this, one may borrow without saying it! We forbid Ribis Me'ucheres, i.e. sending money after the loan 'for your money that delayed with me' (75b). We forbid through a Shali'ach, even without intent for Ribis at the time of the loan.

Defense (Nekudas ha'Kesef): We forbid only when he borrowed directly.

Taz (ibid.): If one vowed not to benefit Ploni, he may not send to him through a Shali'ach. We do not permit due to Ein Shali'ach! Rather, Ein Shali'ach exempts the Meshale'ach from Kares or lashes, but it is forbidden to do so.

Defense #1 (Nekudas ha'Kesef): Regarding a vow, one may not send through a Shali'ach, for the recipient will benefit, and this is forbidden.

Defense #2 (ha'Makneh, Kidushin 42b DH Omar): Ein Shali'ach does not apply here, for the Isur applies only to the one who may not benefit (Ran).

Support (of Taz - Gilyon Maharsha): We do not apply 'Ein Shali'ach' to permit one who made Eruv Tavshilim to cook for one who did not (OC 528:20)!

Taz (ibid.): Through Hefker Beis Din, Beis Din may lend orphans' money in a way more apt to gain than to lose, but may not get absolute Ribis. This is not because Beis Din is the father of orphans (so it is as if the lender directly gave to the borrower). We say so to help them, but not to harm them. Also, if so Beis Din should lend via a Shali'ach and get real Ribis!

Defense (Nekudas ha'Kesef): Regarding orphans, the Gemara gave one of several Heterim.

Levush, Taz and Gra (35): Surely, Rashi never said this. One may not rely on it at all. If I could, I would erase this Heter from Seforim.

Beis Lechem Yehudah and Pischei Teshuvah 17): The Taz is correct. It seems that the argument is not so wide, and he forbids only mid'Rabanan.

Note: The Levush explicitly forbids mid'Oraisa.

Mishneh l'Melech (Hilchos Malveh 5:14 DH ha'Oleh): The Ritva says that a Kohen Shali'ach is not Bar Chiyuva for the Meshale'ach's Kidushin. Likewise, a Shali'ach is not Bar Chiyuva for the Meshale'ach's loan, so Ravina forbids. The Rema rules like Ravina in CM 182. We cannot permit mid'Rabanan Ribis, e.g. through a sale, through a Shali'ach. Perhaps Rashi permits only Ribis mid'Oraisa, for then Ein Shali'ach! The Maharit was unsure about this

Pischei Teshuvah (14): The Sha'ar ha'Melech permits even if Yesh Shali'ach for Isurim mid'Rabanan. Chachamim forbade only Ribis that could lead to Ribis mid'Oraisa!

Pischei Teshuvah (16, citing Mahari Di Boton 168): If the Meshale'ach forced and intimidated the Shali'ach, (Yesh Shali'ach and) he is liable. The Sha'ar ha'Melech disagrees, for the Gemara did not find a pit of partners in this way!

Rema: If a document was written in the name of the lender, primarily, the document makes the loan, and it is forbidden.

Beis Yosef (ibid, citing Maharik 17): If Ploni was a Shali'ach for Reuven to lend to Shimon, and unknown to Reuven he stipulated for Ribis and Shimon signed to obligate himself to Reuven, even Rashi forbids. This is Ribis from the borrower to the lender, from the time he wrote the document.

Gra (37): Writing the document itself is forbidden (62a).