Gemara

18b (Beraisa): All agree that one can sell (his daughter to be a slave, even if she is) a widow to a Kohen Gadol.

Question: How did she become a widow (while a minor)? If she was Mekadesh herself, she would not be called a widow (her Kidushin was void)!

44b (Shmuel (and Rav)): If a minor accepted Kidushin without her father's knowledge, she needs (before marrying someone else) a Get and Mi'un (annulment of a marriage mid'Rabanan).

Objection (Karna): If she needs a Get (i.e. she is Mekudeshes mid'Oraisa), there is no need for Mi'un! If she needs Mi'un, there is no need for a Get!

Answer (Rav Acha brei d'Rav Ika): She needs a Get, lest her father consented to the Kidushin. She needs Mi'un, lest her father did not consent to the Kidushin.

Mi'un makes people suspect that she is not Vadai Mekudeshes, so if Levi (who was Mekadesh her) is Mekadesh her sister, they will not ignore it.

(Ula): Even Mi'un is not needed.

45a (Ravina): Even those concerned lest a man consented to his daughter's Kidushin, are not concerned lest a man consented to his father's Kidushin.

Rabanan: Perhaps the son made his father a Shali'ach!

Ravina: He would not be so brazen to do so.

Question: Perhaps the son told the father that he wanted to be Mekadesh her!

Answer (Rabah bar Simi): Really, Rav and Shmuel would be concerned. Ravina explicitly said that he does not hold like Rav and Shmuel.

45b: David and his wife Sarah argued about whom their daughter would marry. In the end, David agreed that she marry Sarah's relative. During the meal honoring the Kidushin to her relative, his relative was Mekadesh her in another room.

(Abaye): "Yisre'elim will not do unjustly or lie" (surely, David does not consent).

(Rava): One does not make a banquet for naught (if he consented, the Kidushin to her relative was Batel, and the meal was in vain).

If he did not exert himself to make the meal, Rava would be concerned lest he agreed, but Abaye would not.

Rishonim

Rif and Rosh (2:7): Rav and Shmuel say that if a minor accepted Kidushin without her father's knowledge, she needs a Get and Mi'un. Ula does not require even Mi'un. The Halachah does not follow Rav and Shmuel, for Ravina, who is Basra, disagrees. All previous Rabanan, e.g. the She'altos, Bahag and Rav Hai Gaon, say so. Even if he consented, it is not Kidushin. Some are concerned for Rav and Shmuel, for Abaye and Rava argue according to them. I say that their argument does not depend on Rav and Shmuel. They argue about whether we are concerned lest he told her to accept Kidushin in another room. And even if their argument depends on Rav and Shmuel, we rule like Ravina, who is Basra.

Ran (19b DH Garsinan Tu): The Rif holds that Ravina holds that his consent does not help later. He says so because if his consent would make Kidushin, how could we be lenient? It is likely that he consented!

Rebuttal (Rosh 8): They argue only about when we do not know whether her father consented. If he explicitly consented, all agree that she is Mekudeshes! Rashi said 'we are not concerned unless we heard (that he consented).

Question: Ravina is not concerned lest one made his father a Shali'ach only because it is brazen. We should be concerned lest one told his daughter Tze'i v'Kabli (go receive) your Kidushin, if he already agreed that the man will be Mekadesh her!

Answer (Ran ibid.): One is happy that his father do his work for him. It is uncommon to tell one's daughter Tze'i v'Kabli.

Question: Even if consent afterwards does not help, we should be concerned lest the son told his father 'one who will do my will, will be Mekadesh Leah to me', and the father became a Shali'ach for the son by himself!

Answer (Ran): Since it is brazen to make one's father a Shali'ach, when he said 'one who will do my will...' he did not include his father. Indeed, if someone else was Mekadesh her to him, we should be concerned lest he became his Shali'ach in this way! Tosfos says that we are not concerned. According to Rashi, all these concerns are according to Rav and Shmuel. However, we rule like Ravina Perhaps the Rif agrees that immediate consent makes Kidushin. He just says that if he consented later, we are not concerned lest he consented initially. The Rambam holds like this.

Rambam (Hilchos Ishus 3:13): If a girl before Bagrus accepted Kidushin without her father's knowledge, she is not Mekudeshes, even if he consented afterwards.

Rosh (7): The Halachah follows Ravina and R. Simi bar Ashi, for they are Basra. Even Mi'un is not la minor was Mekadesh herself (and her husband died), she is not called a widow (18b). This is like Ravina. This is even if there were Shiduchin, and she did Nisu'in.

Rashba (45a DH v'Rabbeinu): The She'altos proves that the Halachah follows Ravina because we hold that a minor's mother and brothers can be Mekadesh her (to require Mi'un) only after her father died.

Mordechai (515, cited in Beis Yosef DH v'Chosav Od): R. Tam says that Ula discusses only when the father is here and silent. If he is away, we cannot permit her without a Get or Mi'un. Perhaps he will consent! When he is here, Rav and Shmuel say that he consents, and Ula says that he is angry. It seems that we hold like Ravina only when he is here. If he is away, we require a Get and Mi'un, like Rav and Shmuel. When he is here, we cannot ask him, lest he is ashamed to tell the truth because he consented to his wife's choice (like the case of Abaye and Rava). Even though Semag, the Rambam and Bahag did not distinguish whether or not he is here, it seems that we distinguish. Also my father said so.

Poskim

Shulchan Aruch (EH 37:11): If a minor or Na'arah accepted Kidushin without her father's knowledge or had Nisu'in, this is nothing. She does not need even Mi'un. This is even if her father made Shiduchin, and he explicitly consented afterwards.

Beis Yosef (DH veha'Rivash, citing the Rivash (193)): The Rif and Rambam hold that even if he consented immediately, she is not Mekudeshes.

Beis Shmuel (17): Consent at the time of Kidushin (e.g. he was present) works only if Tze'i v'Kabli works.

Shulchan Aruch (ibid.): Some say that if he consented when he heard, she is Mekudeshes from when he heard, even without Shiduchin.

Source (Beis Yosef DH Kosav ha'Rivash): The Rivash (479) says that the Ramah, Rosh and all Acharonim say so, unlike the Rif and Rambam. The Rishonim (Ra'avad, Ba'al ha'Ma'or, Ramach), who elsewhere challenge the Rif and Rambam, accepted their opinion here (Rivash 14) . The Rashba answered the Acharonim's question against them.