Many of those who support the concept of the heter mekhirah today rely (at least in part) on their reading of the Rambam in various parts of the Mishneh Torah, most specifically in Hilkhoth Shemittah WiYovel.
While it is true that the Rambam writes there in 4:28-29 that since goyim aren't commanded regarding shemittah that the produce that grows on land owned by them in Eress Yisra'el is permitted, and that in 10:9 he writes also that shevi`ith in his time (as well as in the times of the Talmudh Bavli and Yerushalmi) is observed by force of rabbinic enactment, but does is this enough to justify the concept?
What about the working of the land in the seventh year by Jews, even if the land is owned by non-Jews? Is this permitted? And what about the Rambam's explicit statements in Hilkhoth `Avodah Zarah 10:1-4 regarding the prohibitions of the misswah of Lo Thehonnem wherein he explicitly and specifically says that the selling of land to non-Jews in Eress Yisra'el is expressly forbidden min ha-Torah? Shouldn't everything that the Rambam states in the Mishneh Torah be taken into account before using his statements to justify the concept of a heter mekhirah?
Can the heter mekhirah truly be supported from the halakhic writings of the Rambam?