If you are Nusach Sefard or Sefardi and are Chazan in an Ashkenaz minyan, or vice versa, as Shaliach Tzibbur you must daven the Nusach of the Shul you are davening in. But what about the quiet Shmoneh Esrei where no one hears and it strictly for yourself? Must you also daven that Tefila in the foreign Nusach of the Shul?
The Shearim Mitzuyanim B'Halacha brings from Rav Moshe Feinstein (Igros Moshe OC 2:29) that you must say the quiet Shmoneh Esrei in the Nusach that you will say Chazaras HaShatz. This he bases on the Mogen Avrohom who says that the Chazan's quiet Shmoneh Esrei is to prepare for Chazaras HaShatz, and if he says it in a different Nusach then he will get mixed up during Chazaras HaShatz. The Shearim Mitzuyanim B'Halacha questions this, since the Rema says that if a Chazan daven from a Siddur he need not prepare the davening beforehand since he can look in and follow. Even the Mogen Avrohom himself says in two places that when davening from a Siddur the Chazan will not make mistakes.
The Shoel U'Meishiv and Netziv both pasken that during the quiet Shmoneh Esrei a person is always obligated to daven his own Nusach regardless of the Nusach he will repeat during Chazaras HaShatz. Furthermore says the Shearim Mitzuyanim B'Halacha, even if the Chazan were to say a few word from the wrong Nusach during Chazaras HaShatz, his Shmoneh Esrei is still valid since the Nusach is not brought down in the Gemara. Additionally he says that Chazaras HaShatz for the past few hundred years is only a Minhag and no one is Yotzei with it anyway.
Important Note: We bring this tshuvah as a starting point for discussion and not to convey any halacha. We try to convey the Tshuva to the best of our ability. We admit that our understanding may not be accurate. One should learn the tshuva to verify the accuracy of our interpretation. Please understand that this Tshuva may not be the final word on this topic. One should consult a Rav before drawing any conclusions.