Gemara

(Rav): The buyer's Keli is used to make Chalipin. He wants the seller to get his Keli, so that the seller will resolve absolutely that the buyer acquire the item.

(Levi): The seller's Keli is used.

(Rava): Levi does not hold that the Keli acquires. Rather, with the pleasure the seller gets that the buyer accepted his Keli, he resolves to give the commodity.

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

If one told Leah 'take this money and be Mekudeshes to Ploni', we learn from (redemption of) a Kena'ani slave that this works. When one pays a slave's master to redeem him, the slave acquires himself (his freedom), even though he does not pay. Likewise, Ploni can acquire a wife without paying.

If Leah told Reuven 'give money to Ploni, and I will be Mekudeshes to him', we learn from the laws of an Arev and a slave that this works.

(Mar Zutra): If Leah told Reuven 'take this money, and I will be Mekudeshes to you,' she is Mekudeshes if he is esteemed. He gives her Hana'ah (pleasure) by accepting her gift. This Hana'ah is Mekadesh her.

Berachos 59b (Beraisa): If one built a new house or bought new Kelim, he blesses Shehecheyanu.

He blesses ha'Tov veha'Metiv only if he has a partner (who shares the benefit).

Rishonim

Rambam (Hilchos Ishus 5:21) If Leah told Reuven 'give money to Ploni, and I will be Mekudeshes to him', Reuven gave, and Ploni said 'you are Mekudeshes to me with the Hana'ah of the gift that I received like you wanted, she is Mekudeshes.

Magid Mishneh: When she herself gives money to Ploni, she is Mekudeshes only if he is esteemed. If not, she received nothing! Here, she loses no money, and benefits from fulfillment of her command with others' money.

Ran (Kidushin 3a DH vHani): An esteemed person does not normally take gifts. She would have paid a Perutah to someone to appease him to accept from her. She received this for free (when he accepted from her), and was Makneh herself.

Question (Ran): In Bava Metzia, Rav holds that a giver does not get Hana'ah when another receives his gift. He did not distinguish whether or not the receiver is esteemed. Levi holds that the giver always gets Hana'ah!

Answer (Ran): Levi holds that there is always Hana'ah, but if the receiver is not esteemed, the Hana'ah is less than a Perutah. This helps for Chalipin, which does not require a Perutah, but not for Kidushin. Rav holds that when an esteemed person receives a gift and will keep it, the giver gets Hana'ah. Normally, one who takes a Kli to do Chalipin gives it back.

Poskim

Shulchan Aruch (OC 223:5): If one received a gift, he blesses ha'Tov veha'Metiv, for it is good for him and the giver.

Magen Avraham (8): The Rosh said that if the recipient is poor, it is good for the giver. He merited to do Tzedakah. If the recipient is rich, the giver is happy that he accepted it. However, there is Hana'ah (for Kidushin - EH 27:9) only if the recipient is esteemed! Many rich people eagerly take gifts. The giver did not benefit! We say similarly in Choshen Mishpat (190:6).

Gra (8): See the [Ran in the] first Perek of Kidushin.

Dameshek Eliezer and Taz (YD 160:8): Kidushin requires Kesef, i.e. a Perutah of Hana'ah; ha'Tov veha'Metiv does not. If the recipient is rich, there is some Hana'ah. It is Shavah Perutah only if he is esteemed.

Machatzis ha'Shekel (DH v'Ayen): The Magen Avraham holds that one may not bless ha'Tov veha'Metiv for a paltry Hana'ah worth less than a Perutah.

Shulchan Aruch (YD 160:15): Some forbid to tell Levi 'tell Ploni to give to you four Zuz, and I will lend to you.'

Beis Yosef (DH Kosav Rabbeinu): R. Yerucham forbids. He learns from Kidushin. If Leah told Reuven 'give money to Ploni, and I will be Mekudeshes to him', we learn from the laws of an Arev and a slave that this works. This case is similar, so we forbid it.

Taz (8): We need not refute this explanation. It is clearly wrong. The Bach says that we cannot forbid when the borrower does not lose! Rather, our text is mistaken. It should say 'tell Ploni to give to me four Zuz, and I will lend to you.' I say that this is obvious; R. Yerucham would not need to teach it. Rather, his law is like Kidushin with the Hana'ah that an esteemed man accepted her gift. She would have paid someone to persuade him to accept. If she tells Levi 'tell Ploni to give to you four Zuz, and I will be Mekudeshes to you', she is Mekudeshes, since Levi was her Shali'ach to tell Ploni, and she wants this. If not for Levi, she would have paid someone else a Perutah to tell Ploni. She benefits from the Shelichus. It is worth money; people do not toil to speak (make such requests) to others for free. This is even if Levi is not esteemed. Surely we forbid her to marry without a Get. I would not rely on this to be lenient, e.g. if later David was Mekadesh her. The same applies to Ribis. We forbid 'I will lend to you if you accept a gift from me' only if the borrower is esteemed and must be appeased to accept gifts. If Levi said 'tell Ploni to give to you four Zuz, and I will lend to you', Levi is happy that he tells Ploni, and he would have paid a Perutah for this, so this is absolute Ribis.

Rebuttal (Gra 31): We need not say like the Taz. The Shulchan Aruch forbids a case like 'give money to Ploni, and I will be Mekudeshes to him.' We learn from the laws of an Arev and a slave that this works. (It is as if she received.)

Mishneh l'Melech (Hilchos Malveh 5:14 DH Kosav Rabbeinu): One may not say 'give money to Ploni, and I will lend to you', even though the lender does not receive. One may not tell David 'give money to Ploni, so he will lend to me', for David is like the borrower's Shali'ach. Just like we combine these laws regarding Kidushin, also here. One may not tell David 'tell Reuven to give to Ploni, and I will lend to you.' Ploni's receiving is as if the lender received. Reuven's giving is as if David gave. The Levush forbids to tell David 'give money to Ploni, and I will lend to him', or to tell the borrower (Levi) 'tell Ploni to give money to you, and I will lend to you.' His first law is wrong. There is no Ribis! Granted, in his second law Levi's receiving is like the lender's receiving. In the first case, it is as if the lender received, but the borrower did not lose. The Levush misunderstood the Beis Yosef.

Mekor Mayim Chayim: The Magid Mishneh says that when she loses no money, she is Mekudeshes even if he is not esteemed. Sha'ar De'ah says that likewise here, the lender gets proper Hana'ah when the borrower receives according to his command, and it is forbidden due to Ribis. This supports the Levush's first case! However, the Shulchan Aruch brings only the second case. This shows that he disagrees with the first.

Shach (20): Since the lender wants the borrower to get the gift, the borrower may not tell Ploni, for his Shelichus benefits the lender.