After the burial of Sarah, Avraham presents his servant Eliezer with the task of finding a wife for Yitzchak The Torah introduces this section with the Pasuk (24:1), " VeAvraham Zakein Ba BaYamim, VaHashem Beirach Et Avraham BaKol," "Avraham was old, well on in years, and Hashem blessed Avraham with everything." Rashi comments that the word "BaKol" has the same Gematria (numerical value) as the word "Ben," a son. According to this understanding, the Torah opens this section by pointing out that Avraham had a son, so he needed to find a wife for him.
The Gemara (Bava Batra 16b) offers a different interpretation of this Pasuk. Chazal say that Avraham had a daughter named "Bakol." From where did Chazal come to this conclusion? The Chachamim resolve this issue by explaining a Machloket in Masechet Yevamot regarding the Mitzvah of Peru Urvu. Beit Shammai says that in order to fulfill this Mitzvah, one must have two sons. This is derived from Moshe Rabbeinu, who had two sons with his wife Tziporah and then left her (see Bamidbar 12:2 and Rashi there). On the other hand, Beit Hillel says that one fulfills this Mitzvah by having one daughter and one son. This is derived from the Pasuk (Bereishit 5:2), " Zachar UNekeivah Beraam," "Male and female He (Hashem) created them." Comparable to creation, one fulfills the Mitzvah of Peru Urvu by having a daughter and a son. The Halacha was established in accordance with the view of Beit Hillel. Since Chazal say that Avraham fulfilled every Mitzvah of the Torah, they assume that he fulfilled the first Mitzvah of the Torah, that of Peru Urvu. Avraham therefore must have had a daughter, and her name was "Bakol."
Before Matan Torah, it was permitted to marry one's sister. Therefore, why didn't Avraham arrange a marriage between Yitzchak and Bakol? One of the Baalei HaTosafot suggests that Bakol was too young to marry Yitzchak. However, it was only after Akeidat Yitzchak that Avraham learned of Rivkah's birth (for presumably, that's when she was born). Sarah had died after she heard Yitzchak was almost slaughtered. As a result, Bakol must have been older than Rivkah. If so, Bakol's age could not have precluded her marriage to Yitzchak, since it did not preclude Rivkah's marriage to Yitzchak.
The Zera Berech explains that after Akeidat Yitzchak, there was no doubt whom Yitzchak was going to marry. Although there was a major age difference between Yitzchak and Rivkah, Hashem still notified Avraham that Yitzchak would marry Rivkah. The Baal HaTosafot was dealing with this issue before Akeidat Yitzchak, when Avraham was unaware of Rivka's imminent birth. Therefore, Avraham didn't marry off Yitzchak to Bakol because he believed she was too young for him. Only once Rivkah was born did Avraham realize that the age difference was not an issue, but at that point he had already been told that Yitzchak would marry Rivkah.
In the end, everything is in Hashem's hand. We may plan and calculate, but Hashem will ultimately decide what is going to happen. As we say in Shacharit (Mishlei 19:21), "Many designs are in man's heart, but only the council of Hashem will prevail."