Gemara

(Mishnah): If Levi promised to give a certain amount to his son-in-law, and before the Nisu'in he stuck out his foot, his son-in-law can say 'I will not make Nisu'in until you give the money. Your daughter will be unable to marry anyone else';

Admon (and R. Gamliel) say, she can claim 'had I myself promised, this would be proper. My father promised. What should I do? Make Nisu'in with me, or divorce me!'

(Gemara): Our Mishnah is unlike the following Tana:

(Beraisa #1 - R. Yosi b'Rebbi Yehudah): Admon and Chachamim agree that if a man promised money to his son-in-law, and stuck out his foot, she can say 'my father promised. What should I do?'

They argue when she promised. Chachamim say that her husband can say 'I will not make Nisu'in until you give the money';

Admon and R. Gamliel say, she can say 'I thought that my father would give. Now that I see that he will not, what can I do? Make Nisu'in, or divorce me!'

(Beraisa #2): This applies when she is an adult. If she is a minor, we force.

This cannot mean that we force her father. It is more reasonable to force him if she is an adult!

(Rava): It means that we force her husband to divorce her.

(R. Yitzchak ben Elazar): Wherever R. Gamliel agreed with Admon, the Halachah follows Admon.

Question (Rava): Does this even apply to the Beraisa?

Answer (Rav Nachman): He did not say 'in the Mishnah.' He said 'wherever'!

Rishonim

Rif: We hold like the Tana of our Mishnah. Nowadays that it was enacted to give a Get to a rebel, whether her father promised or she did, we force her husband to divorce her, against his will.

Ran (DH Masnisin): The Ra'avad says that 'stuck out his foot' means that her father died, or went overseas. If he was here, since these promises are acquired verbally, we would collect from his property. It seems that he did not promise at the time of Kidushin, for then all would agree that she can force him to make Nisu'in or divorce her, since he can force her father to pay. Rather, he promised at the time of Shiduchin. We can say that the father refuses to pay, or has no money. The Yerushalmi says that if she was not present when her father promised, Chachamim agree that we force her husband. The Mishnah discusses when she was present, and Admon argues.

Beis Yosef (EH 52 DH Kosav): Rabbeinu Meir says that the Bavli holds oppositely. If she was present, it is as if she herself promised.

Rebuttal (of Rif - Hasagos ha'Ra'avad and Ba'al ha'Ma'or): The Mishnah connotes that had she herself promised, we would not force her husband. Rav Nachman said that we follow Admon wherever R. Gamliel agreed with him, i.e. even in Beraisa #1, i.e. when she promised. Further, due to the enactment of a rebel, the Rif did not need to say that the Halachah follows our Mishnah!

Question: How can Rav Nachman rule like Beraisa #1 and the Mishnah? The Gemara said that our Mishnah is unlike the Beraisa!

Answer #1A (and defense of Rif - Ran DH v'Garsinan): Surely, we do not follow this Beraisa against the Mishnah! Rather, Rav Nachman means that if we find another Beraisa in which R. Gamliel supports Admon, the Halachah follows Admon.

Answer #1B (Milchamos Hash-m): Rav Nachman refers to a Beraisa in Shevu'os (40b). If Reuven claimed an ox and a Seh and Shimon admitted to one of them, he must swear. R. Yochanan said that the Beraisa is like Admon. Rav Nachman knew a Beraisa in which R. Gamliel agrees with Admon. Also R. Chananel and BaHaG rule like Admon in our Mishnah.

Note: The Gemara there said that the Beraisa cannot be like R. Gamliel, for R. Gamliel obligates even in the Reisha (Reuven claimed an ox, Shimon admitted to a Seh), and the Beraisa exempts!

Answer #2 (Tosfos DH v'Afilu): Rav Nachman means that wherever we find an opinion that they argue, i.e. when her father promised (in the Mishnah), or when she promised (in the Beraisa), the Halachah follows Admon.

Answer #3 (Bach EH 52 DH u'Mah she'Chosav v'Yesh): The Beraisa does not contradict the Mishnah. In the Beraisa, she promised in front of her father, therefore she can say that she expected her father to pay. In the Mishnah, she was not present, like the Yerushalmi says. Therefore she says 'had I promised, this would be proper.' She cannot expect her father to pay what she promised in his absence! (At first, our Gemara said that the Beraisa argues, because it held that the Mishnah discusses when she was present.)

Ran (DH Gemara): Perhaps the Beraisa discusses even if her father was not present when she promised. Alternatively it is only when he was present. If not, Admon would agree that we do not force her husband.

Rambam (Hilchos Ishus 23:16): If Levi promised to give money to his daughter and he went overseas, she can say 'my father promised. What can I do? Either make Nisu'in without a dowry, or divorce me! However, if she promised, she waits until she attains what she promised, or until she dies. She cannot exempt herself through rebellion, for that is when he wants Nisu'in and she does not. Here, he does not want Nisu'in without the dowry, and she wants Nisu'in, for she says 'make Nisu'in or divorce me!' This refers to an adult. If a minor promised for herself, we force him to divorce her or make Nisu'in without a dowry.

Rosh (13:11): We hold like the Tana of our Mishnah. Some say that since Rav Nachman said (that the Halachah Admon when R. Gamliel agrees) everywhere, he holds like the Tana of the Beraisa. The Halachah follows them.

Beis Yosef (EH 52 DH Kosav): The Ran (64a DH Garsinan) and Rosh (5:35) hold that nowadays we do not abide by the enactment of a rebel. This is why the Rosh did not comment on what the Rif said about this.

Poskim

Shulchan Aruch (EH 52:1): If Levi promised money to his daughter and he went overseas, or he lacks the money, she can say 'I did not promise for myself. Either make Nisu'in without a dowry, or divorce me!' However, if she promised, she waits until she attains the money, or until she dies. This refers to an adult. If a minor promised for herself, we force him to divorce her or make Nisu'in without a dowry.

Chelkas Mechokek (1): The Shulchan Aruch connotes that she was present at the time, like the Mordechai.

Beis Yosef (DH v'Chosav): The Rambam says that if she was a minor, we force him. He understands that Beraisa #2 (that distinguishes minors from adults) explains the Mishnah. The Rif omitted this, for he holds that Beraisa #2 explains Beraisa #1, and he holds that we do not follow Beraisa #1. The Rosh also omitted it. This shows that also he rules like the Mishnah. This is unlike the Tur, who says that the Rosh rules like Beraisa #1.

Rebuttal (Bach DH Aval): The Rosh understands that the Beraisa #2 explains Chachamim in Beraisa #1. He omitted it, for he holds that we follow Admon in Beraisa #1, and we force her husband in every case.

Rema: This is when she lacks the money to give what her father promised. If she has it, she must give it. She cannot say 'make Nisu'in without a dowry, or divorce me!' This is before Nisu'in. After Nisu'in, he cannot betray her just because he does not get what he was promised. He must fulfill every obligation of a husband.

Chelkas Mechokek (3): It seems that if she had the money to give what her father promised, but now she does not, she cannot say 'what can I do.

Bach (DH Kosav): Shiltei ha'Giborim (64a:1) says so (if she has the money, we do not force him) regarding the Beraisa, when she herself promised. Hagahos Maimoniyos (Teshuvah 28 on Hilchos Ishus) says so even when her father promised. Even though after Nisu'in he cannot betray her, he can force her to pay, even if her father promised. This requires investigation.