Nosson Tzvi, it is really great to try and answer your questions! -Especially since you were Zocheh to be Mechaven to the question of the Acharonim!

1) First, I should point out that the Tosfos ha'Rosh in Kidushin 19b (end of DH Mashal) writes that since the Halachah follows the opinion that the original money was given for Kidushin (unlike the opinion of Rebbi Yosi b'Rebbi Yehudah on 19a), it is as if the master said at the time of the sale "Mei'Achshav, from now, whenever I want, I will perform Yi'ud." The Tosfos ha'Rosh adds that this means that if someone had relations with her from the time of the sale until the time of the Yi'ud, this is considered a violation of the prohibition of having relations with a married woman, because the Yi'ud applies retroactively from the time of the sale. According to this, not only is the money given earlier, but the Kidushin also takes place earlier.

2) The Or Same'ach (Hilchos Avadim 4:15) explains why there is no proof from the Mitzvah of Yi'ud that the Halachah follows the opinion that Yesh Bereirah. He compares our Sugya to the Gemara in Eruvin 36b. The Mishnah there discusses the case of a person who wants to hear a Shi'ur from a Chacham on Shabbos but he does not know whether the Chacham will come to the east side of the city or to the west side. He puts down two Eruvei Techumin and says, "If the Chacham comes to the west side, I will use the Eruv on the west side, and if he comes to the east side, I will use the east Eruv." The Gemara states that the status of the Eruv in this case depends on whether the Halachah is Yesh Bereirah or Ein Bereirah. However, Rebbi Yochanan states (end of 36b) that if the Chacham has already arrived and the person wanting to attend his shiur simply is not yet aware of which side of town the Chacham has arrived to, the Eruv does not depend on the question of Berierah because it has already been decided; it is already "Nisbarer."

3) The Or Same'ach writes that we learn from Rebbi Yochanan that the question of Bereirah does not apply to an event which has already happened. Similarly, in our Sugya of Yi'ud, the Kidushin has already happened. This is because it is as if the master, who buys the Amah Ivriyah, says to her, "You are hereby married to whoever owns the money of Kidushin." Afterwards, we will know whether the Kidushin money belongs to the father or to the son. However, the Kidushin happens immediately, as soon as the master gives the money to the father of the Amah. The master says to the father of the girl, "If I will want to give the girl to my son through Yi'ud, then the money belongs to him now." The latter statement does not depend on Bereirah but is a straightforward Tenai (condition) with only one side (namely, if I will want to do Yi'ud for my son). There are not two possibilities, and thus it is not related to Bereirah.

4) The Or Same'ach cites Tosfos in Beitzah 5a (DH v'Niskalkelu) as another source for the concept that if something has already happened then Bereirah does not apply. The Gemara's discussion there concerns Rosh Hashanah in the time of the Beis ha'Mikdash when the witnesses might arrive in the middle of the day and proclaim that they have seen the new moon, making that day Rosh Hashanah. What Korban is brought on that day (before witnesses comes), when it is not known whether Rosh Hashanah is today or tomorrow? Tosfos writes that they brought the Korban Tamid, which is brought on every weekday, and made a condition that "if witnesses do not arrive today, this will be a Korban Tamid, but if witnesses do arrive, it will be a Korban Musaf." How does this condition work according to the opinion of Ein Bereirah? The Or Same'ach answers along the lines of the above reasoning. In this scenario, the concept of Bereirah is not relevant, because if the witnesses arrive today this means that retroactively it was Rosh Hashanah from the beginning of the day. As we said above, Bereirah does not apply for something which has already happened.

5) This is the common factor. If the witnesses come today, this means that Rosh Hashanah has already happened. Similarly, if the father does Yi'ud for the son, this means that right from the beginning, the father of the girl received the money from the son. Therefore, Bereirah does not apply.

Kol Tuv,

Dovid Bloom

Here is a different approach given by the Acharonim about the relationship of Yi'ud to Bereirah.

1) Rav Isser Zalman Meltzer zt'l writes in Even ha'Ezel (Hilchos Avadim 4:7, DH Achen) that even if one says that the original money of Yi'ud is given for Kidushin, this does not mean that the Kidushin works retroactively, because if so the Gemara should have said that if she was unfaithful between the sale and the Yi'ud, that she must bring a Korban Chatas.

2) The Even ha'Ezel adds that another reason for why one must say that the Kidushin does not work retroactively is the well-known question from Bereirah about the way Yi'ud works. To answer your question, he writes that when we say that the original money was given for Kidushin, this means that the money acquires for the father the right later on to perform the Yi'ud either for himself or for his son, but the Kidushin comes into effect only afterwards. The Chidush of the Torah of Yi'ud is that the father of the girl can give over to the master the right to perform Yi'ud either for himself or for his son, and it follows that Bereirah is not relevant here.

3) The Even ha'Ezel continues and says that after he wrote the above, the Chidushim of his teacher, Rav Chaim Solovetchik zt'l, were published on the Rambam to Hilchos Yibum v'Chalitzah. Rav Chaim also wrote (4:16 DH v'Hineh) that Yi'ud does not mean that the Kidushin works retroactively. However, Rav Isser Zalman writes that Rav Chaim's explanation is slightly different from his, because according to Rav Chaim the action of Kidushin is effected at the beginning, when the money is given over to buy the Amah, while the completion of the Kidushin is dependent on what the master decides later on -- whether she will be married to him or to his son.

4) At any rate, the explanation of Rav Chaim and of Rav Isser Zalman is clearly not consistent with the Tosfos ha'Rosh that I cited in my previous reply, that the Kidushin works retroactively and she is considered a married woman right from the beginning. However, it may be that Rashi does not learn like the Rosh. See Rashi (19b, DH me'Achshav) who writes that the Kidushin starts when the money is paid by the master to the father of the girl. (Rashi (19b, DH Ha and DH l'Rabanan) also writes that the Kidushin starts when the money is paid). Possibly there is a proof from Rashi for what Rav Chaim writes that the Kidushin starts only when the money is given over, and is not completed until the father decides who will be the husband. Rashi appears to be different from the Rosh, since according to the Rosh everything is finished right at the beginning when the money is given (see Birkas Avraham to 19b).

5) According to the above approach, we may also say that since the Kidushin is not completed when the first money is given, it follows that the question from Bereirah is not a question.

Kol Tuv,

Dovid Bloom

Here is yet a different answer from the Achronim to your important question.

1) The Chazon Ish (Even ha'Ezer #148, to Kidushin 19a DH LM'D)discusses which features of the Mitzvah of Yi'ud are a Chidush, a new idea, which one does not find in standard marriages. The Chazon Ish writes that according to the opinion "Ein Bereirah" one must say that Yi'ud is a "Chidush" because the fact that the father can do Yi'ud either for himself or for his son is consistent only with the opinion of "Yesh Bereirah."

2) The Chazon Ish is clearly referring to the idea that we find in the Gemara that one cannot transfer a Halachah from a Mitzvah of the Torah which is a "Chidush" to other areas of the Torah ("mi'Chidush Lo Gamrinan"). For instance, see Pesachim 44b where the Gemara states that one cannot derive the Halachah of Ta'am k'Ikar (that the taste of a forbidden food is prohibited even if only the taste is present without any visible prohibited substance) from the prohibition of meat and milk, because the prohibition of meat and milk is a "Chidush" since if one soaks meat all day long in milk it is not forbidden mid'Oraisa, but if one cooks meat with milk it is forbidden by the Torah. Rashi (DH Chidush) writes that the entire Mitzvah of meat and milk is surprising and unusual (because meat by itself is permitted, milk by itself is permitted, but the two together are forbidden), and therefore one cannot derive anything from it to other areas of Halachah.

Similarly, the Chazon Ish writes that even though one does indeed see from Yi'ud that "Yesh Bereirah," nevertheless one cannot compare this to the rest of the Torah because Yi'ud is different from the other Mitzvos of the Torah.

3) In fact, we also find in the Rishonim the idea that Yi'ud is a Chidush, such as in the Ritva to 18a (DH Mahu). The Gemara there cites a Beraisa which lists ways in which it is easier for a Hebrew male Eved to go free than it is for a Hebrew female Eved. The Gemara challenges the Beraisa from a different Beraisa which states that the female Eved has more ways of going free than the male, namely because she goes free when she develops physical signs of maturity. The Gemara answers that when the first Beraisa states that the male goes free more easily than the female, this refers to a girl who did Yi'ud. The Gemara asks that if did Yi'ud, then it is obvious that the male can go free more readily than she, because she would now require a Get. The Gemara answers that one might have thought that since she was an Amah Ivriyah, we do not abolish for her the standard ways that an Amah Ivriyah goes free and she would not require a Get. This is why it is necessary for the Beraisa to say that she does need a Get.

4) The Ritva writes that the reason why one might have thought that a Kidushin can be dissolved without a Get is that the Kidushin of Yi'ud is very different from the standard Kidushin, since it can be performed without the agreement of the father. We never find elsewhere that Kidushin can take effect without agreement on the side of the bride. The Ritva writes that one must conclude that it is a Gezeiras ha'Kasuv that Yi'ud can work against the regular rules. The Ritva writes that since this is the case, "Ein Lecha Bo Ela b'Mah she'Amrah Torah," -- "One cannot learn from this more than what the Torah stated explicitly." This phrase of the Ritva is similar to a concept mentioned sometimes in the Gemara, "Ein Lecha Bo Ela Chidusho" (see, for example, Sanhedrin 27a) -- if there is a Din in the Torah which is unusual, one cannot extend it any further than what is stated explicitly in this Din.

5) Therefore, according to the Chazon Ish, even though Yi'ud can work only according to the concept of Bereirah, nevertheless one cannot derive from this that Bereirah applies in other areas of the Torah, because Yi'ud is an unusual Mitzvah which cannot be compared to other Dinim of the Torah.

Pesach Kasher v'Same'ach,

Dovid Bloom