Gemara

6b (Abaye): If one was Mekadesh with a loan, she is not Mekudeshes. If one was Mekadesh with Hana'as Milveh (benefit of a loan, i.e. he granted an extension), she is Mekudeshes, but one may not do so, for it is a scheme for usury.

47a (Rav): If a man was Mekadesh a woman with money he previously lent to her, she is not Mekudeshes, for a loan is given to be spent.

(Beraisa): If a man was Mekadesh a woman with a loan, she is not Mekudeshes. Some say that she is Mekudeshes.

Suggestion: They argue about Rav's law. The first Tana holds that a loan is given to be spent. The latter Tana says that it is not.

Rejection (Seifa): All agree that this acquires regarding a sale.

If a loan is given to be spent, how can it acquire?!

(Rav Huna): Rather, in the Beraisa he said that he is Mekadesh her with 100 coins, and he gave only 99. The first Tana holds that she so she does not agree to 99, for she is ashamed to ask for the last coin. The latter Tana says that she is not ashamed. (She accepts 99, and expecting to ask for the last one later).

48a (Beraisa - R. Meir): If one said 'be Mekudeshes to me with this document', she is not Mekudeshes;

R. Elazar says, she is Mekudeshes;

Chachamim say, we evaluate the paper. If it is worth a Perutah, she is Mekudeshes.

Suggestion: It is a document for a loan that she owes to him. The Tana'im argue about whether or not a loan can make Kidushin.

Rejection (Rav Nachman bar Yitzchak): No, he gave her a Shtar Kidushin without signatures. R. Meir disqualifies such a document. R. Elazar validates it if it was given in front of witnesses. Chachamim are unsure whom the Halachah follows. If it is worth a Perutah, she is Vadai Mekudeshes. If not, she is Safek Mekudeshes.

(Beraisa - R. Meir): If one said 'make this metal into chains and rings for me, and I will be Mekudeshes to you', once he makes them, she is Mekudeshes;

Chachamim say, she is Mekudeshes only when she gets (other) money.

Suggestion: They argue about whether or not a loan (what she owes him for his labor) can be Mekadesh.

Rejection: No, all agree that a loan cannot Mekadesh. They argue about whether or not wages accrue continuously (if they do, they are like a loan).

Rishonim

Rif (2b): How can Hana'as Milveh be better than a loan itself? There is Hana'ah of (an extension on) a loan. The loan itself is not intact, for it is given to be spent.

Rif and Rosh (19b and 2:8): Kidushin with a loan does not take effect.

Ran (2b DH Omar and 20a DH v'Garsinan): Kidushin with a loan is invalid. We learn from Sedei Efron. He must give to her something at the time of Kidushin. Even if the money is intact, it was given to be spent; she owes different money. She need not invest it so that it will be available whenever he demands to be paid back. Therefore, it is like hers, and he did not give to her anything. Hana'as Milveh is Kidushin 'with the Perutah you would have paid someone to appease me to grant you an extension.' All the more so, this Hana'ah of pardoning the loan works. If one was Mekadesh with the money itself, he gave nothing. She does not intend to be Mekudeshes through the Hana'ah of his pardon, rather, through the money. Perhaps even when he said 'be Mekudeshes through the Hana'ah of my pardon', she intends for the money, and it is invalid.. When he gives to her another's loan she is Mekudeshes, for she intends for the Hana'ah, not for the money.

Rambam (Hilchos Ishus 5:13): If one was Mekadesh with a loan, even if there was a document, she is not Mekudeshes. E.g. if she owed him a Dinar, and he said 'you are Mekudeshes to me with the Dinar you owe me', she is not Mekudeshes, for a loan is given to be spent, and there is nothing existing now to benefit from. She already spent the Dinar, and its benefit passed.

Beis Yosef (EH 28 DH u'Mah she'Chosav Afilu Adayin): The Rambam means that since a loan is given to be spent, even if she still has the money, it is as if she already spent it, and there is nothing existing now to benefit from.

Maharit (Chidushim on Rif, middle of first Perek, DH Garsinan): The Rambam holds that if the money is around, she is Mekudeshes. Even though it was already in her Reshus, when she accepts the loan for Kidushin, she relinquishes to him her rights to the money, in order that he can be Mekadesh her. This is like one who stole from a woman and was Mekadesh her with it. She is Makneh the stolen item to him so that the Kidushin can take effect.

Beis Shmuel (EH 28:19): The Rambam connotes that if she still has the money, she is Mekudeshes. However, the entire Sugya connotes otherwise.

Poskim

Shulchan Aruch (EH 28:7): If she owed to him a loan, even if there was a document, and he said 'you are Mekudeshes to me with the Dinar you owe me', she is not Mekudeshes, even if the money is in her hand and she did not use it.

Rema: This is even if he returned the document to her and it is worth a Perutah.

Beis Yosef (DH u'Mah she'Chosav Afilu Hichzir): We are concerned for the document itself only if he said 'be Mekudeshes to me with this document.'

Magid Mishneh (Hilchos Ishus 5:13): The Rambam did not mention this. If he held that she is Mekudeshes, he should have mentioned this when he discussed returning a security for the loan.

Rema: Some are stringent in this case.

Taz (15): This opinion equates this case to Kidushin with a loan and a Perutah.

Chelkas Mechokek (18): They hold that if the document is worth a Perutah, she is Mekudeshes. If it is not worth a Perutah, she is Safek Mekudeshes, lest it is worth a Perutah elsewhere.

Gra (24): The Gemara said that if her document is worth a Perutah, she is Mekudeshes. Tana'im argue only about if it is not worth a Perutah. Even though the borrower pays the scribe (so it is her document), it is like a security of the lender, therefore he pardons to her a loan with a security.

Shulchan Aruch (ibid.): The same applies if she owes him wages for working for her, even if the payment for the loan or wages is due.

Beis Yosef (DH u'Mah she'Chosav): We learn from Kidushin 48a. Even if she owed him for making rings, she is not Mekudeshes.