מקדש במלוה ה׳

(13) When [a man] consecrates [a woman] with a debt, even with [a debt that is recorded] in a promissory note,17The Ramah (Even HaEzer 28:7) notes that if the promissory note is worth a p'rutah and he returns it, there are opinions that maintain that the consecration is binding. she is not consecrated.
What is implied? [The woman] owed [the man] a dinar; if he tells her, "Behold, you are consecrated to me with the dinar that you owe me," she is not consecrated. [The rationale is that] a loan is given to be spent, and there is nothing that presently exists for her to derive benefit from [and to accept as kiddushin]. For she has [- or it is as if she has - ]18I.e., even if she has not actually spent the money, from the time she received the loan, the money is hers and not the lender's, and he cannot consecrate her with it (Beit Yosef, Even HaEzer 28). See also Beit Shmuel 28:19. already spent that dinar and has derived benefit from it already.

(15) When [a man] consecrates [a woman] with the benefit [derived from] a loan, the consecration is valid.
What is implied? The consecration is binding if he lends her 200 zuz [at the time of the kiddushin] and tells her: "Behold, you are consecrated to me through the benefit [you receive] by my extending the length of this loan for you. It may be in your possession for so many days, and I will not demand payment until this date." For she is receiving benefit now [from the opportunity] to use the loan until the end of the time period fixed.
It is forbidden to make [such a condition], because it is like taking interest.20Since in addition to the eventual repayment of the debt, the person also receives the benefit of consecrating the woman, it is regarded like interest. The Rabbis (Meiri, Ma'aseh Rokeach) explain that the Rambam's wording is precise. The expression "like interest" implies that it is not actually considered to be taking interest, as forbidden by Scriptural law. My teachers interpreted the expression "the benefit [derived from] a loan," in a way that is not worthy of mention.21The Rambam is referring to Rabbenu Yitzchak Alfasi, who interprets the passage from Kiddushin 6b as referring to a person who extends the length of a loan at the time that payment is due. The Rambam does not accept that interpretation, because it is not logical that extending the length of the loan would be more effective than forfeiting the debt entirely (Maggid Mishneh).
Rabbenu Yitzchak Alfasi's view is also followed by Rashi and the Ra'avad. The Shulchan Aruch (Even HaEzer 28:9) quotes the Rambam's interpretation (for even the opinions that differ agree that such kiddushin are binding). In the law that follows, it also quotes the opinion of Rabbenu Yitzchak Alfasi. Although the opinion of the Rambam is mentioned, the other view is favored. The Ramah, however, considers the status of the kiddushin to be doubtful because of the Rambam's view.