I wanted to mention that the major difference between Rav Shach (Laws of First Fruit, chapter 2, law 13) and Reb Chaim of Brisk in terms of the question in the Rambam about the difference between Chalah and Maasar and the reason and owned by a gentile is not obligated in truma even according to the opinion a gentile does not take away that obligation is this. Reb Chaim want to equate truma and maasar with hekdesh in that you need both the third growth and the smoothing to be in the possession of a Jew. Thus, land owned by the gentile is not obligated in truma until the Jew buys it back and the produce reaches a third of its growth in the possession of the Jew--even though this last condition is not spelled out in the Rambam. Rav Shach is explaining the Rambam according to the Keseph Mishna in so far that you can explain the Rambam either like Rabah (there is no possession) or like R Elazar (there is possession) For example if there is no possession and the land is obligated in truma even in the hands of the gentile but not until the Jew buys it back. But in any case, there is no requirement of the third growth to be in the possession of the Jew. This is because according to Rabah there is no requirement of the third growth and Rav Shach brings a proof of the idea and support to the Keseph Mishna from Bava Batra page 81 in Tosphot that explains even though there is no possession to a gentile still because of the verse your land the land of the gentile is not obligated until the Jew buys it back. --------------------------------------I wanted to mention that the major difference between רב שך and רב חיים of בריסק in terms of the question in the רמב’’ם about the difference between חלה andמעשר and the reason and owned by a gentile is not obligated in תרומה even according to the opinion a gentile does not take away that obligation is this. ר' חיים want to equate תרומה and מעשר with הקדש in that you need both the third growth and the smoothing to be in the possession of a Jew. Thus, land owned by the gentile is not obligated in תרומה until the Jew buys it back, and the produce reaches a third of its growth in the possession of the Jew , even though this last condition is not spelled out in the רמב’’ם. NOW רב שך is explaining the רמב''ם according to the כסף משנה in so far that you can explain the רמב’’ם either like רבה (there is no possession) or like ר' אלעזר (there is possession) For example if there is no possession and the land is obligated in תרומה even in the hands of the gentile, but not until the Jew buys it back. But in any case, there is no requirement of the third growth to be in the possession of the Jew. This is because according to רבה there is no requirement of the third growth and רב שך brings a proof of the idea and support to the כסף משנה from בבא בתרא page פ''א in תוספות that explains even though there is no possession to a gentile still because of the verse "your land" the land of the gentile is not obligated until the Jew buys it back.