What do we learn from "Asher Nishbati b'Api"?
Chagigah 10a: This is a source to permit Nedarim [and Shevu'os]. I swore in my anger, and I retract.
Sanhedrin 110b #1: Dor ha'Midbar has no share in the world to come. (I swore that they will not come to My Menuchah.)
Sanhedrin 110b #2: Dor ha'Midbar has a share in the world to come. (I swore in my anger, that they will not come to My Menuchah, and I retract.)
Radak: Due to this (they were "To'ei Levav" and "Lo Yad'u Derachai"), I swore.
Why does it say "Im"?
Radak: This is like a Shevu'ah, like one who says 'so and so will be, if this occurs.' We find "Im Yir'u Es ha'Aretz Asher Nishbati la'Avosam" (Bamidbar 14:23).
What is "Menuchasi"?
Rashi: It is Eretz Yisrael and Yerushalayim, which I called Menuchah - "Zos Menuchasi Adei Ad" (Tehilim 132:14).
Radak: It is Eretz Yisrael - "El ha'Menuchah 1 v'El ha'Nachalah" (Devarim 12:9), for Yisrael rested there. Menuchasi is the Menuchah that I gave to them. This is like "v'Simachtim b'Veis Tefilasi" (Yeshayah 56:7) - the Tefilah that they pray in front of Me.
So expounds Sifri Devarim 2:2. The Gemara expounds Menuchah to be Shilo or Yerushalayim. (PF)
