What it looks like to me is that the Rambam is like the second answer in Tosphot.
I am still in the middle of Rav Elazar Menachem Shach's essay on this Rambam.
[In laws of the seventh year in the Rambam.] But just off hand it looks to me that the area of Amon and Moav that the Gemara says is obligated in tithes to the poor on the seventh year is talking about the areas that were not conquered by the Jews coming out of Egypt.
That seems fine. But what that would mean for some city in side of Israel proper that was conquered by the Jews coming out of Egypt but not by the Jews coming back from Babylonia is that a city like Beit Shan in the Galil would have to give tithes to the poor on the seventh year and also be obligated in the laws of the seventh year. And that seems contradictory.
I mean to say that the laws of the seventh year would be obligatory from the Torah just by the fact of it being the land of Israel even though it was not sanctified in by the Jews coming back from Babylonia. But on top of that it would have a rabbinical obligation to give מעשר עני tithes to the poor. What is wrong with this is tithes to the poor is dependent on the order of the years. It is given on the third and sixth year. And even if there would be some rabbinical decree to give on the seventh year but the fruits are open for all to take anyway!
Appendix: Land that was conquered by Jews coming from Egypt but not by Jews coming back from Babylonian is not worked but the fruits growing by themselves can be eaten after the ביעור. That seems to imply it is obligated in the seventh year laws from the Torah itself.--Even though it was not sanctified! OK that is unexpected, but reasonable. But what makes it hard to understand it that the very same land would be obligated in tithes to the poor! What could that mean? The fruit is anyway open for anyone to take.
That is land that is עולי מצרים but not עולי בבל is נאכל but not נעבד. The Rambam might have explained that like the Rabainu Shimshon. But he did not. He said it means ספיחים. So we have שביעית from the Torah even though קדושה ראשונה קדשה לשעתה ולא קידשה לעתיד לבא. And in spite of that, it still in obligated in מעשר עני! How does this fit?
I think Rav Shach might be asking the same thing. For what I remember when learning this with my learning partner is the Rav Shach focuses on the fact the land is not worked so how is מעשר עני applicable? Maybe that is the same thing I am asking?