Parshas Va’eschanan begins with Moshe davening to be allowed to enter Eretz Yisrael. Hashem told Moshe to stop davening, for He had made a vow that Moshe would not enter Eretz Yisrael. Rashi tells us that Moshe had thought that the vow was annulled because he was allowed to conquer and enter the land of Sichon and Og, which were annexed to Eretz Yisrael and divided among the shevatim of Reuven, Gad and part of Menashe.
The status of the land in Eiver Hayarden is not clear. On one hand, it was not part of the “promised land.” When Hashem promised the Avos that their children would inherit “this land” it did not include the land on Eiver Hayarden. Yet regarding certain aspects, land on Ever Hayarden is considered part of Eretz Yisrael. The Mishnah in Bikurim (1:10) says that min haTorah fruits grown in the land on Eiver Hayarden obligated in terumos and ma’aser. This implies that it is indeed a part of Eretz Yisrael.
The Ramban in Parshas Matos (31:23) says that the reason why the Torah only taught the halachos of koshering utensils after the war with Midyan and not after the war with Sichon and Og, was because the war against Sichon and Og was fought to conquer Eretz Yisrael. The Gemara in Chullin 17a says that during a war waged to conquer Eretz Yisrael the soldiers could eat non-kosher. Therefore it was not relevant to teach the halachos of koshering utensils when they could eat actual non-kosher food. The Ramban there points out that based on this, the land of Sichoin and Og is considered part of the conquest of Eretz Yisrael. The Ramban then asks how Moshe was in fact allowed to enter that land. He leaves the question unresolved.
The Mishnah Lamelech in his sefer Prashas Drachim quotes the Mahari Ade’Rebi who explains that Moshe only thought that the land was part of Eretz Yisrael and therefore thought to beseech Hashem to allow him to enter the rest of Eretz Yisrael. However, in fact the land of Sichon and Og is not part of Eretz Yisrael.
The Mishnah Lamelech questions this approach because of the above-mentioned mishnah in Bikurim that implies that the land of Sichon and Og were in fact part of Eretz Yisrael. Additionally he cites Rashi (Devarim 18:1) who quotes the Sifri that says that kohanim and levi’im who are not to take a portion of land for themselves, are equally restricted from possessing land in Ever Hayarden. This too implies that the land in Ever Hayarden is part of Eretz Yisrael.
The Chasam Sofer answers the apparent contradiction. He explains that the land of Sichon and Og on the Ever Hayarden in fact became part of Eretz Yisrael, but only after Bnei Yisrael conquered Eretz Yisrael proper. Therefore, when Moshe conquered the land of Sichon and Og it did not yet have the status of Eretz Yisrael. However the reason why that land is obligated in terumos and ma’aser and the kohanim and levi’im cannot posses land in those countries is because after Bnei Yisrael did conquer Eretz Yisrael it did become kadosh as part of Eretz Yisrael.
In my opinion, we find a similar concept regarding the arei miklat (cities of refuge) that Moshe set up in Ever Hayarden. They did not provide refuge until the cites of refuge were activated in Eretz Yisrael.
However this does leave us with another question. Why would Moshe Rabbeinu have thought that the vow that disallowed him to enter Eretz Yisrael was annulled simply because he was allowed to conquer and enter the land of Sichon and Og? At that time those lands did not posses any kedusha. Why would he believe that the promise was affected?