How could Avrohom have considered burying Sarah not in the ground?

The Ramban writes on the posuk (23,8) "If it is your will that I bury my dead from before me", that the explanation of the words "from before me" is that Avrohom said to them: If you do not do sell me a burial plot, then I will bury her in a coffin.

But this is a problem according to what the Ramban himself writes in his sefer Toras HaAdam from the conclusion of the gemara in  Sanhedrin 46b, that according to the Torah the Mitzvah of burying the dead can only be fulfilled by burying the body actually in the earth. But burying the body in a coffin is included in the negative commandment not to leave the body unburied overnight, and the Mitzvah to bury the dead is not fulfilled by this at all. But from Avrohom's declaration - that if they do not give him land for a burial plot, then he will not bury her in the earth at all, but instead he will bury her in a coffin - it seems that also with this one can fufil the Mitzvah of burial.

But it seems to my humble opinion that we can explain it according to what I wrote elsewhere to explain the posuk in Devarim (34,6) concerning Moshe's death, "and no person knows his place of burial, until this day", that burial the dead is for one of two reasons. One of them is that which is brought in the Midrash on the posuk in Bereishis (3,19) when Hashem said to Adam haRishon after he had sinned, "With the sweat of your face you shall eat bread, until you return to the ground, for from it you were taken; for dust you are, and to dust you will return". Hashem said to him: The handful of earth from which you were created, is it not stolen property in your hand? The explanation of this is that man was created from the ground in order to rectify it and purify it, so that it will be like the soul. But since Adam sinned and did not rectify the earth, he must return to the ground. And this is like returning a stolen article to its owner, as the Maharal of Prague explained.

And I remember that one I was young I heard my Rebbe from Socatchov answer with this the gemara in Chullin 92b, which writes that one of the Mitzvos which the Noachides still observe, is that they do not weigh out the flesh of a dead person in the marketplace, see Rashi there. But where do we see that Noachides are commanded not to eat a dead man's flesh? The Rav from Socatchov answered that since the Mitzvah to bury the dead in the ground is returning a stolen article to its owner, and someone who does not do so is stealing from the ground, therefore eating a dead man's flesh is in the category of stealing, and Noachides are commanded not to steal.

And the sefer Toras HaAdam also brings the gemara which says that Rebbi commanded that when they bury him, the coffin should have a hole in it. What he meant was that they should remove the bottom panel of the coffin and lay him on the earth, because it is a Mitzvah to bury in the earth; and not because the earth of Eretz Yisrael was so beloved to him, but even outside of Eretz Yisrael, as it is written, "and to dust you will return".

The second reason why we have to bury the dead, is that which the Tosafos Yom Tov wrote in the second chapter of Pirkei Avos, Mishnah 7, in the name of the Kol Bo, because it is hard for the soul to see the body disgraced, lying around without burial.

The practical difference between these two reasons is if the body is buried in a coffin. According to the second reason he will have fulfilled the Mitzvah of burial, since the body is not being disgraced. But according to the first reason of returning a stolen article, we require burial in the actual earth, and this cannot be completely fulfilled with a whole coffin in the ground, as the Ramban wrote.

Now, Moshe Rabbeinu was called a man of G-d, and was purified completely like a ministering angel. Therefore, burial in the earth was not relevant to him, since he was no longer of the same type as the earth at all, and so there was no stolen article to return. And this is the explanation of "and no person knows his place of burial", because why did he need to be buried in the ground when it was sufficient to bury him in a coffin.

And now the words of the Ramban regarding the burial of Sarah make good sense, because Chazal taught that Sarah died without any sin. If so, she had rectified the material part of herself completely, and there was no longer any relevancy of her body to the ground, and no aspect of returning a stolen article. Therefore, according to everybody the Mitzvah of burial according to the Torah could be fulfilled by burial in a coffin.

Now, the gemara Sanhedrin 46b learns that the Torah requires burial in the ground from the double expression כי קבור תקברנו - "but you shall surely bury him". That is, if the Torah had only written קבור I would have thought that burial can be fulfilled with a coffin, and therefore it wrote the superfluous word תקברנו to teach that we require burial in the actual ground. We can explain that the reason that the gemara does not learn this from that which Hashem said to Adam "until you return to the ground", is because we cannot bring a proof from the law of the Noachides before the giving of the Torah, to the law of Yisrael after the giving of the Torah, because the Torah was given to them in order to purify their material selves through the power of the Torah, and to make their earthly portion like the soul. - Indeed, this is why Chazal taught that only you (Yisrael) are called Adam (alluding to the fact that they came from the ground), because by Yisrael it is a worthy appellation, indicating that even though that we are created from the ground, nevertheless, through the power of the Torah we can become purified like the angels. But this is not so by the nations, and so by them the name Adam would be a derogatory term, and therefore they are not to be called Adam. - If so, there is good reason to think that after the giving of the Torah burial in the ground is not required, and burial in a coffin is sufficient for the reason of the Kol Bo. Therefore, an extra word was needed to teach us otherwise.

Now, the opinion of R. Chananel is that the Mitzvah of burial is only from the Rabbis. We must say that he means that burial in the ground is only from the Rabbis, and according to this opinion there is no proof from Adam HaRishon to after the giving of the Torah, because it was given to Yisral to elevate their material selves to the level of the soul, and therefore they do not indeed require burial in the ground according to the Torah.

And we can explain with this a puzzling Midrash on the posuk "The sea saw, and it fled". What did it see? The coffin of Yosef. The commentaries had much difficulty with this Midrash. But we can explain it using also what the Sforno wrote on parshas Vayechi, that Yosef was buried in a coffin, and not in the ground. And it seems that this does not conflict with the gemara in Sotah 13, which brings a dispute whether the the coffin was sunk in the River Nile, or buried in the burial plot of the Kings. Because certainly according to the opinion that it was sunk in the Nile, burial in the ground was not fulfilled. But even according to the other opinion, since it was a whole coffin made of metal, the Mitzvah of burial in the ground was not fulfilled.

And perforce the reason for this is because Yosef was a complete Tzaddik, and had rectified his material portion completely so that he no longer had any relevancy to the ground. If so, he was above nature, and therefore nature had no right to block his way, and so when the sea saw him, it fled from before him.