QUESTION: The Gemara mentions different opinions as to how one should conclude the blessing of Havdalah. The Tana'im in a Beraisa argue whether one should say "Soder Bereishis" ("the Organizer of Creation") or "Yotzer Bereishis" ("the Maker of Creation"). Rebbi Yosi b'Rebbi Yehudah says that the appropriate conclusion should be "Mekadesh Yisrael" ("the Sanctifier of Israel").
Rav rules like Rebbi Yosi b'Rebbi Yehudah, that one should say "Mekadesh Yisrael." Shmuel rules that one should say, "ha'Mavdil Bein Kodesh l'Chol" (which is the common practice). Even though this is not the opinion of any of the Tana'im in the Beraisa, Shmuel follows the opinion of another Tana, as the Rashbam (DH u'Shmuel) explains.
The Gemara relates that Abaye "cursed" anyone who followed Rav's opinion. What upset Abaye so much that he had to curse the one who says "Mekadesh Yisrael" at the end of Havdalah?
(a) The RASHBAM explains that one who says the phrase "Mekadesh Yisrael" implies that Yisrael is the only thing in the world which Hash-m sanctified. This implication is inaccurate. Other things in the world are also sanctified (such as the Beis ha'Mikdash). Perhaps this is what upset Abaye.
(b) RAV ELAZAR MOSHE HA'LEVI HOROWITZ suggests a different answer. At one point in history, the Nochrim decided that the seventh day of the week, the Sabbath, should be moved to Sunday. If a person mentions holiness, Kedushah, on Motza'ei Shabbos and says "Mekadesh Yisrael," the Nochrim might use that as proof that they are the true people of Israel and that Sunday is the day that is holy. Therefore, Abaye did not want that phrase to be used in Havdalah on Motza'ei Shabbos.
This hypothesis also explains why Abaye "cursed whoever follows the opinion of Rav," even though the Tana, Rebbi Yosi b'Rebbi Yehudah, used Rav's formula in his Havdalah. In the days of Rebbi Yosi b'Rebbi Yehudah, the Nochrim had not yet disseminated their corrupt ideas, so at that time there was no concern if one were to end Havdalah in such a manner.
QUESTION: Rebbi Yehoshua ben Levi rules that when one recites Havdalah, he should mention in the blessing only the forms of separation that appear in the Torah.
However, the Gemara says that when Yom Tov occurs on Motza'ei Shabbos, one says in the blessing of Havdalah, "ha'Mavdil Bein Kodesh l'Kodesh," in order to express that Hash-m separated between the Kedushah of Shabbos and the Kedushah of Yom Tov. Where does the Torah mention a separation between the holiness of Shabbos and the holiness of Yom Tov? The Torah does not even make explicit mention of the separation between the Kedushah of Shabbos and the lack of Kedushah of the rest of the week. (See RASHBAM, DH v'Chosem.)
(a) It seems from the Rishonim that Rebbi Yehoshua ben Levi's requirement applies only to the main text of Havdalah. The blessing at the end of Havdalah, though, is not limited to what is mentioned explicitly in the Torah.
(b) The ME'IRI offers another answer. He asserts that the Torah does distinguish between two types of Kedushah. The verse says that in the Mishkan, the purpose of the Paroches was to separate between the Kodesh and the Kodesh ha'Kodashim (Shemos 26:33). Just as the Torah explicitly differentiates between those two forms of Kedushah, it also differentiates between the Kedushah of Shabbos and the Kedushah of Yom Tov.
(Similarly, the Me'iri learns that the words in the blessing of Havdalah that is recited every Motza'ei Shabbos, "ha'Mavdil Bein Kodesh l'Chol," also have a basis in the Torah. Although there is no specific mention in the Torah of a separation between Shabbos and Chol, there is mention of a separation between Kodesh and Chol (Vayikra 10:10). That verse expresses the distinction between Shabbos and Chol.)