Rav Shach suggest that the Rambam has a different approach to the two gemaras in Rosh Hashanah 13 and and Avoda Zara 23 . The one in Avoda Zara we know he holds with since that is the source of the idea that if someone sets up a brick to worship and then someone else comes along and worships it then it is forbidden even though a person can not forbid that which belongs to someone else. Still in this case the first person has already revealed his acquiescence.
But the Gemara in Rosh Hashanah, the Rambam simply does not hold with. The reason is this. It says How could Israel bring the Omer after they arrived in the land of Canaan? Did not the grain grow in the possession of gentiles? And the Omer has to be brought from a harvest that belonged to a Israeli.
In fact the Rambam does not bring that law that the Omer has to be brought from a harvest that belonged to a Israeli. Rather he holds like the Gemara in Avoda Zara that the land belonged to Israel from the time of Abraham.
[But Rav Shach is not simply saying that the two gemaras disagree with each other סוגיות חלוקות. Rather that the Gemara is Rosh Hashana 13:a hold with the opinion יש קנין לנכרי בארץ ישראל להפקיע מתרומות ומעשרות and so even though everyone holds that the land of Canaan belongs to Israel from the time of Avraham still they should not have been able to bring the Omer since the grain that grew in the possession of a gentile would anyway not be obligated in tithes. But the law is אין קניין לנכרי להפקיע מידי תרומות ומעשרות and the law does not follow the Gemara in Rosh Hashana.
