Gemara

(Mishnah 1): If one bird of a Ken Stumah (it was not designated which bird will be the Chatas and which the Olah) flew away, we take another bird to replace it, to be offered with the remaining bird.

Nazir 12a (R. Yochanan): If Reuven made David a Shali'ach to be Mekadesh an unspecified woman and David died, Reuven may not marry any woman.

There is a Chazakah that a Shali'ach fulfills his mission. Since Reuven did not specify, we do not know whom David was Mekadesh. (Perhaps he was Mekadesh the sister of the woman Reuven wants to marry now!)

Question (Reish Lakish - Mishnah): If one bird of a Ken Stumah flew away, we take another bird to offer with the remaining bird.

Inference: Had the birds been specified, the remaining bird could not be offered. (This is not in the text of Tosfos and the Rosh. They say that the same applies if the birds were specified.)

(Summation of question): We are not concerned about other Kinim (that one of the birds is the bird that flew away, and cannot be used for a different Korban). Similarly, we should not be concerned lest the Shali'ach was Mekadesh Leah's relative!

Answer (R. Yochanan): Because women do not roam (their dignity is to stay in the house), the Safek is Kavu'a (fixed), so it is considered an even doubt, and we are stringent. Birds roam, so we follow the majority.

(Rava): R. Yochanan agrees that Reuven may marry Leah if she has no (unmarried) daughter, granddaughter, mother, grandmother or sister.

Even if she has a sister (or another of these relatives), but the sister was divorced (or widowed) after Reuven appointed the Shali'ach, Leah is permitted to Reuven.

Question: We should be concerned lest David was Mekadesh her sister to Reuven!

Answer: One authorizes a Shali'ach (with intent) only for something that can be done at that time. (David was not a Shali'ach to be Mekadesh the sister, for she was married at the time.)

Rishonim

Rif, Rosh and Rambam (Kidushin 21b, 2:18 and Hilchos Ishus 9:6): If Reuven made David a Shali'ach to be Mekadesh an unspecified woman and David died, Reuven may marry Leah if she has no relative (that would forbid her to Reuven), and even if she has a relative, but the relative was divorced after Reuven appointed the Shali'ach. This is because one authorizes a Shali'ach only for something that can be done at that time.

Note: The Rosh says 'one can authorize a Shali'ach only for something that can be done at that time.'

Magid Mishneh: Tosfos asked that every woman should be forbidden to marry! (Perhaps David was Mekadesh her!) If she is believed to say 'I did not become Mekudeshes', Reuven should be permitted to any woman whose relatives say that they did not become Mekudeshos! Also, we should forbid any minor girl who had a father, and the father died or went overseas. Perhaps the Shali'ach was Mekadesh the minor through her father! Tosfos answered that the Isur is a fine for doing improperly, to make a Shali'ach to be Mekadesh a woman Stam.

Ramban (Gitin 64a Hashmatos 21 DH Ha): Even orphaned minors are permitted, because they had a Chazakah of being single. The Chazakah that a Shali'ach fulfills his mission does not override the Chazakah of being single. Reuven is forbidden to all women, because the Chazakah of relatives, who do not come in front of us to rule about them, does not help for him. If they would come and testify that they were not Mekudeshos, surely they are permitted, for one witness is believed about Isurim, especially here that there was no Chezkas Isur and it is a mere concern. Since they are believed about themselves and may marry others, all the more so they are believed about him.

Magid Mishneh: Some learn from here that one can be a Shali'ach to bring Kidushin to his relative, even though could not be Mekadesh her himself. If not, Reuven would be permitted to David's relatives!

Poskim

Shulchan Aruch (EH 35:11): If Reuven made David a Shali'ach to be Mekadesh an unspecified woman and David died, and Reuven does not know whether or not David was Mekadesh anyone, the Chazakah is that he was Mekadesh, for there is a Chazakah that a Shali'ach fulfills his mission. Since we do not know whom David was Mekadesh, Reuven is forbidden to any woman Leah with relatives that forbid Leah to him due to Ervah, e.g. her sister or daughter. Reuven may marry a woman without such relatives. If Leah has such a relative, but the relative was married at the time Reuven made a Shali'ach, even if she was divorced before David died, Reuven may marry her. We are not concerned lest David was Mekadesh her after she was divorced, because it was not feasible to be Mekadesh her when he made a Shali'ach, and one authorizes a Shali'ach only for a woman that he could be Mekadesh at that time.

Beis Yosef (DH veha'Ran): The Ran (Gitin 30a DH v'Neima) says that the Ramban is difficult. He holds like Tosfos.

Chelkas Mechokek (23): It is not a proper Chazakah, for it is not b'Yado (in the Shali'ach's power). Perhaps she does not want to be Mekudeshes to Reuven! Even so, we are stringent to consider it a Chazakah.

Beis Shmuel (28): We apply the Chazakah to be stringent, but not to be lenient. We apply it to be lenient about Eruvin, for Eruvin is only mid'Rabanan. The Taz says that this (the Chazakah) is why we do not apply to a Sefek-Sefeka to be lenient. Perhaps he was not Mekadesh, and even if he was, perhaps he was not Mekadesh this woman. This is wrong. We do not rely on a Sefek-Sefeka when one Safek is about the matter itself, and one Safek is whether the Safek was about this element of the mixture.

Beis Shmuel (29): We are stringent only for relatives that create a Torah Isur, but not for Sheniyos (Isurim mid'Rabanan). Rashi, Chelkas Mechokek and the Prishah say so, unlike the Taz.

Rema: Similarly, if Leah's relatives say 'we did not become Mekudeshos', and they married, Reuven may marry Leah, for surely her relatives did not become Mekudeshos.

Chelkas Mechokek (25): This is only if the relatives married, but not if they merely said that they are single. Tosfos and the Ran explain that we fine Reuven for entering such a Safek. He should have commanded him to be Mekadesh a particular woman.

Rema: Some say that similarly, if Reuven made David a Shali'ach to be Mekadesh Plonis, and David died, Reuven may not marry Plonis' relative due to the Chazakah that a Shali'ach fulfills his Shelichus.

Beis Yosef (DH v'Chasav): The Rivash (82) learns from here that if Reuven made David a Shali'ach to be Mekadesh Plonis, he is immediately forbidden to her relatives, due to the Chazakah. This is if it is possible that David was Mekadesh her relative, unless her relative says 'I did not become Mekudeshes.' If the Shali'ach says 'I was Mekadesh Plonis', and she denies it, Reuven is forbidden to Plonis' relatives, for his Shali'ach is like him, but Plonis is permitted to Reuven's relatives.

Chelkas Mechokek (26): Surely, in this case we are stringent for the Chazakah. If above we forbid him to all women in the world, all the more so here he is forbidden to relatives of the woman he commanded about! Here, Tosfos and the Ran agree to the Rambam that it suffices for Plonis to say 'I did not become Mekudeshes.' Here there is no reason to fine him.

Beis Shmuel (30): Ramban argues with Tosfos about a minor whose father went away, and also in any case that Leah's relatives testify that they did not become Mekudeshos. If it is a fine, this does not permit Reuven. Also, if Reuven was Mekadesh a woman with a sister, mother or daughter, according to Tosfos it is full Kidushin. According to the Ramban, it is only Safek Kidushin, for the Chazakah creates a Safek that he was already Mekadesh the relative.

Gra (25): According to the Ran, it suffices if the relatives became Mekudeshos to others.

Rema: Some disagree

Chelkas Mechokek (27): Darchei Moshe says that Tosfos disagrees. Tosfos says that the Isur is due to a fine. This implies that if he designated a particular woman to be Mekadesh, we do not forbid him. Darchei Moshe erred. Tosfos said 'we fine for commanding to be Mekadesh a Stam woman, without concern lest he stumble in sin. Letter of the law he is permitted to all women. We do not forbid him to all women, for that is a great stringency. The Shali'ach was Mekadesh only one woman. How can we forbid all women?! It is a fine not to follow the majority. Also, the Chazakah is not a full Chazakah, for it is not b'Yado (perhaps she will not consent). Therefore, it is only a fine to forbid all women. When he specified one woman, letter of the law her relatives are forbidden. This is not a fine. It is not a great stringency to say that a Shali'ach fulfills his mission, even though it is not b'Yado. The Gemara asked that we should rely on the Chazakah to be lenient about a Get!

Beis Shmuel (31): Chelkas Mechokek is correct. Also the Taz says so. Even Tosfos agrees that when he commanded to be Mekadesh Plonis, she is believed to say that the Shali'ach was not Mekadesh her.