The commandment of the 3rd rabbinic commandment compared to the eighth root in the Book of Mitzvot by Maimonides.

Rabbi Michael, peace and blessings

My name is S., and I teach at the Machanim Yeshiva in Gush Etzion.

I am currently dealing with the Ramban's interpretation of the verse 'And the land shall not be sold forever,' and comparing his interpretation in the Babylonian Makhot 3:2 with his interpretation in the Torah. It seems that there is a great difference between the two sources, which even reaches a significant halachic conclusion.

In doing so, I would also like to address the Ramban's interpretation of the commandment of the "compiler of the commandments" in the Book of Mitzvot. The Rambam listed as a mitzvah the prohibition of selling land in Israel in a way that it will not return in the Jubilee. This is one of the mitzvahs not listed by the Rabbis.
The Ramban explains that the Rabbi refrained from this part of the mitzvah, 'because in his opinion negations do not prevent it,' and this is proof of his opinion that the Rabbi understood perfectly the division presented in the eighth root, 'and is not, as the Rabbi suspected, not understanding the matter of negations at all.'

The questions I am trying to answer are as follows:
A. Does the Ramban himself agree with the Ha-Hag that there is no such thing, or does he, after justifying the Ha-Hag's method, still support the idea that there is a nothing?
on. What is the practical ruling when a person sells a field with a "not to be returned in the year of the jubilee"? This is a twofold question – 1. If there is a no, does he cancel the sale? 2. If there is no no, is it because it is not valid in any case?

This raises a fundamental question for me about the eighth root in general:
In your detailed discussion in the book 'Yishlach Sharshiyo' you addressed the negation regarding the dimension of 'permission', which is neither a prohibition nor an obligation. However, is there a negation that says that there is no prohibition in something simply because it is not possible? For example, there is a prohibition against incest that focuses on the act of marriage between people who are forbidden from each other. However, to the best of my knowledge, we have not found a prohibition against marrying one's sister, etc., because the marriage is not valid at all. The commandments of the Torah that refer to marriage are relevant only in the case where the marriage itself is valid.

I would be very grateful for any assistance.

Thank you very much.