Where Does The Kedusha Of The Land Come From

The Rambam in the beginning of the Laws of Terumot (1:3) says הָאֲרָצוֹת שֶׁכָּבַשׁ דָּוִד חוּץ לְאֶרֶץ כְּנַעַן כְּגוֹן אֲרַם נַהֲרַיִם וַאֲרַם צוֹבָה וְאַחְלָב וְכַיּוֹצֵא בָּהֶן אַף עַל פִּי שֶׁמֶּלֶךְ יִשְׂרָאֵל הוּא וְעַל פִּי בֵּית דִּין הַגָּדוֹל הוּא עוֹשֶׂה אֵינוֹ כְּאֶרֶץ יִשְׂרָאֵל לְכָל דָּבָר וְלֹא כְּחוּצָה לָאָרֶץ לְכָל דָּבָר כְּגוֹן בָּבֶל וּמִצְרַיִם. אֶלָּא יָצְאוּ מִכְּלַל חוּצָה לָאָרֶץ וְלִהְיוֹתָן כְּאֶרֶץ יִשְׂרָאֵל לֹא הִגִּיעוּ. וּמִפְּנֵי מָה יָרְדוּ מִמַּעֲלַת אֶרֶץ יִשְׂרָאֵל מִפְּנֵי שֶׁכָּבַשׁ אוֹתָם קֹדֶם שֶׁיִּכְבּשׁ כָּל אֶרֶץ יִשְׂרָאֵל אֶלָּא נִשְׁאַר בָּהּ מִשִּׁבְעָה עֲמָמִים. וְאִלּוּ תָּפַס כָּל אֶרֶץ כְּנַעַן לִגְבוּלוֹתֶיהָ וְאַחַר כָּךְ כָּבַשׁ אֲרָצוֹת אֲחֵרוֹת הָיָה כִּבּוּשׁוֹ כֻּלּוֹ כְּאֶרֶץ יִשְׂרָאֵל לְכָל דָּבָר. וְהָאֲרָצוֹת שֶׁכָּבַשׁ דָּוִד הֵן הַנִּקְרָאִין סוּרְיָא.  The indication is had Dovid conquered it after the rest of Eretz Yisroel then it would be part of Eretz Yisroel.  The Radvaz asks why, true that כיבוש may work to create an obligation of Terumot, but how do we know that lands not promised to Avrohom Avenu can have the same level of kedusha as the rest of Eretz Yisroel?  It is clear from the question of the Radbaz that he assumes what gives kedusha to the land of Eretz Yisroel is the promise to avrohom Avenu, not an act of conquest.  It would seem that he goes l'shitaso in the Laws of Sanhedrin (4:6.)  The Rambam says that what defines Eretz Yisroel regarding the law that semicha can only be administered in Eretz Yisroel is the land that was conquered when we entered the land the first time, after leaving Egypt.  The Radvaz notes that even if it wasn't conquered in the second Temple era and it lacks the obligation of the mitzvot of the land (see Rambam Terumot 1:5,) it still will have kedusha of Eretz Yisroel affecting the law of semicha, the mitzvah to live in the land etc.  We see he holds that it is the act of conquest, כיבוש that creates the halachik obligation on the land but the spiritual, intangibles, the kedusha of the land, emanates from the promise to Avrohom Avenu (see more about this in Mishnas Yaavetz Yoreh Deah siman 26.) 

The Rambam in Shabbos (6:11) rules that one may tell a gentile to write a bill of sale that he is buying the land from him on the Shabbos, because the rabbis didn't impose the prohibition of telling a gentile when it comes to the mitzvah of yishuv Eretz Yisroel.  The Rambam rules the same applies to Suryah.  This ruling stems from Gittin (8b.)  The issue is that the Gemorah there is going in the opinion that כיבוש יחיד is a valid כיבוש, however, the Ramban that rules in the Laws of Terumot that it doesn't constitute a halachik כיבוש, so how does he rule that there is yishuv Eretz Yisroel regarding Suryeh as well?  The simple approach would be that yes, כיבוש יחיד isn't valid vis a vis the obligations on the land but kedusha there is.  It is the kedusha of the land that determines if their is a mitzvah of living on the land and that is present in Suryah.  However, this flies directly in the face of the Ridvaz that the kedusha eminates from the proise to Avrohom (or maybe from the first כיבוש,) however, Suryeh which wasn't part of that shouldn't have kedusha at all?  What is interesting to me is that the Mahari Kurkos seems to suggest this idea in the Rambam while at the same time mentioning that the kedusha stems from the promise to Avrohom.  If that's the source of it, how can it apply to Suryah?

[This whole assertion of the Radvaz would seem to be debated by the Rishonim in Gittin (2a) if there can be kedusha to the land even if the land wasn't conquered by the second Temple era, see תוס' ד"ה ואשקלון וברמב"ן וריטב"א and I'm too lazy to elaborate further.]

Another point of interest is the Magen Avrohom (306:20) when questioning the Rambam in a similar vein, assumes the heter to tell the gentile to write the bill of sale is only to avoid a negative of לא תחנם, not the possitive of yishuv Eretz Yisroel.  If that's the case, even if one where to entertain that Suryah had kedusha, there still is no negative prohibition in selling one's house to a gentile there and the answer would be out the window.  However, I don't know why the Magen Avrohom assumes this way when the terminology of the Gemorah and the Rambam is that the אמירה לעכו"ם is permitted for the positive mitzvah of yishuv Eretz Yisroel?  Furthermore, in my ignorance, I don't understand how does the lav of לא תחנם permit the issur, לא תחנם is not to sell the house to the gentile in the first place, however, here it is already sold to him so you aren't saving yourself from any prohibition?  Unless I am misunderstanding the lav?

As a point of clarification, when I said the kedusha emanates from the promise to Avrohom, I didn't mean that it started then.  It could very well be it has special kedusha from creation, but I'm focusing on kedusha as it applies to our activities of yishuv Eretz Yisroel and other such laws.