Gemara

(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: The Tana'im argue about whether or not a loan (what she owes him for his labor) can be Mekadesh. All hold that wages accrue continuously, from the start of the job until the end, so they are like a loan that she owes to him. R. Meir holds that a loan cannot Mekadesh, Chachamim say that it can.

Rejection #1: No, all agree that a loan cannot be Mekadesh. They argue about whether or not wages accrue continuously;

R. Meir considers it as if all the work is done at the moment it is completed (he gives her new money, not a loan). Chachamim hold that wages are owed (at each moment, according to how much has been completed) from the start until the end, so it is like a loan that she owes to him.

Rejection #2: All agree that wages accrue continuously, and that a loan cannot be Mekadesh. They argue about whether or not Uman Koneh bi'Shvach Kli (a worker acquires improvements to a Kli on which he is working).

R. Meir holds that he acquires. (The wages are not like a loan. Rather, it is as if he sells the Kli when he finishes.) Chachamim disagree.

Rejection #3: All agree that Ein Uman Koneh bi'Shvach Kli, and that wages accrue continuously, and that a loan cannot be Mekadesh. The case is, he added (at least a Perutah) to what she gave to him, and also pardoned her debt to him. They argue about whether she intends to become Mekudeshes through his addition (this works), or through the loan, which does not work.

Rishonim

Rif (Bava Kama 35a): We conclude that all agree that Ein Uman Koneh bi'Shvach Kli. The Halachah does not follow Rav Asi.

Question (Ran Kidushin 20a DH Garsinan): According to the opinion that Ein Uman Koneh, and wages accrue continuously, he make keep the Kli until he is paid. Kidushin with a loan with a security takes effect, like R. Yosi b'Rebbi Yehudah taught!

Answer (Ran): Some say that this is why the Rif omitted R. Yosi b'Rebbi Yehudah's teaching. This is wrong. The Rif (2b) brought it! Rather, a lender acquires a regular security due to R. Yitzchak's teaching. Here, the Uman does not acquire it, for he did not receive it for a security. A Mishnah (Bava Metzi'a 80a) says that an Uman is a Shomer Sachar. We say that this is unlike R. Yehudah, even though R. Yehudah agrees that one who lent Peros is a Shomer Sachar on the security (for he benefits, i.e. he will not lose if the Peros rot). The same applies to an Uman, for he is happy to be hired! Also, Rav Sheshes says that one who hired a craftsman transgresses Bal Talin. If an Uman acquires the item like a security, surely Bal Talin would not apply! Rather, it is not a security until he demands payment and seizes the item for payment.

Rosh (Bava Metzi'a 9:43): Rav Sheshes taught that Bal Talin applies to Kablanus, because Ein Uman Koneh. The Halachah follows him, for a Beraisa supports him.

Rambam (Hilchos Ishus 5:20): If one said 'you are Mekudeshes with this work that I will do for you', she is not Mekudeshes unless he added a Perutah of his own. This is because a worker acquires wages from the start until the end. Whenever he does part of the job, he acquires part of the wages. Therefore, all the wages are a loan, and Kidushin with a loan is invalid.

Poskim

Shulchan Aruch (EH 28:15): If one said 'you are Mekudeshes with this work that I will do for you', she is not Mekudeshes.

Beis Yosef (DH Lefichach): When he returns the work to her, retroactively it is a loan. Since Ein Uman Koneh, it is as if she owes to him wages.

Prishah (36): Because wages accrue continuously, he is Mekadesh her with a loan. According to the opinion that wages are not owed until the end, even if he does not return the Kli, she is Mekudeshes at the end. They agreed on this at the beginning.

Rebuttal (Beis Shmuel 38): According to the opinion that we0, he must give the Kli to her to be Mekadesh her. Tosfos (Bava Kama 99a) explicitly says that even the Hana'ah of his pardon cannot be Mekadesh until he returns the Kli.

Shulchan Aruch (ibid.): Some say that she is Mekudeshes.

Rema: This is if he is a Kablan (he contracted to do the entire job for a set price), for Uman Koneh bi'Shvach Kli. If he still has the Kli and was Mekadesh with his wages, all agree that she is Mekudeshes, for this is like a loan with a security.

Chelkas Mechokek (27): All agree that a loan with a security is Mekadesh (Sa'if 11). One opinion says that this is even before he returns the security, for he intends to return it. This is like R. Yerucham, who says that the argument about whether or not Uman Koneh is if he already returned the Kli and afterwards says 'be Mekudeshes to me with the wages you owe me.' If he is Mekadesh at the time he returns it, all agree that she is Mekudeshes. This is unlike the Ran, who say that it is like a security only if he seized it after claiming his money. The Bach cites the Ritva in the name of the Ramban to say that even if he seized it after claiming his money, it is unlike a security for a loan. Why did the Rema rule like R. Yerucham against the Ramban, Ritva and Ran? At least he should say that it is Safek Kidushin!

Beis Shmuel (40): The Ramban holds that it is like a security only if it was given for a security. The Tur and R. Yerucham hold that whenever he says that he seizes it for a security, it is a security. The Ran holds that this helps only if he already claimed his wages.

Bach (DH Kosav): The Tosfos Rid says that the Mekadesh must say at the time of Kidushin that he is Mekadesh her with the wages for the Kli. If he said so only beforehand, she is not Mekudeshes. This is like one who said 'you are divorced with the Get that I will give you.' It is valid only if he says so when he gives it to her. This is obvious.

Chelkas Mechokek (27): I say that one must bring a proof for this!