Thinking about the question of whether there was a mitzvah for Pharoah to listen to Moshe triggered my wondering about a somewhat related question. Rashi cites derasha after derasha in explaining the halachos of korban pesach defined by the pesukim in our parsha, Shmos ch. 12. The Netziv takes note of the Torah’s language in emphasizing (12:28) that Bnei Yisrael did “ka’asher tzivah Hashem es Moshe” – the term “tzivah” as opposed to “ka’asher diber” is inclusive of Rabbinic explication in addition to the words G-d spoke. Yet, if the basis for Rabbinic authority is the mitzvah of “lo tasur” (as the Rambam writes), there seems to be no compelling reason for Bnei Yisrael to have accepted Moshe’s interpretation and derush of these pesukim, as the mitzvah of “lo tasur” was yet to be given. Why then did they listen?
We already discussed a similar question in the past. The gemara (Shabbos 87) tells us that as a result of Moshe’s interpretation of Hashem’s command, he added an extra day into the calculation of when the Torah would be given. R’ Elchanan asks: Granted that this interpretation was done through Moshe’s use of the 13 midos of torah sheba’al peh, but what authority or basis did those midos have prior to the commandment of “lo tasur’ being given? I don’t understand – why is R’ Elchanan bothered by this gemara and not bothered by the derashos galore on our parsha? I think I must be missing something but can’t figure out what.