Gittin 47a - What constitutes a "hafka'ah" from mitzvos?

Tosafos discusses the precise definition of being "mafkia" the land of Israel from Ma'aser. Tosfaos proves that R' Meir (avoda zara 21a) holds that non-jewish ownership is exempt from ma'aser, because R' Meir says that selling land in Israel to a goy is "mafkia" it from ma'aser. Tosafos explains that the hafka'ah from ma'aser cannot simply mean that it is chayev in ma'aser, but the goy won't be doing it. Rather R' Meir means that by selling to a goy the land will no longer be chayev in ma'aser, therefore the sale is considered a hafka'ah from ma'aser. Tosafos proves that "hafka'ah" must mean uprooting the chiyuv, rather than simply not performing the mitzvah from the continuation of the sugya in Avoda Zara. The gemara asks that by a house also it should be assur to sell to a goy since you are "mafkia" the house from the mitzvah of mezuza - clearly the gemara did not think that a goy's house is chayev in mezuza just that he won't be doing it, rather the gemara knew that the house of a goy is exempt. So we see from the gemara's question by mezuza that the definition of hafka'ah is to put the house in a situation where it is going to be exempt from mezuza, similarly by the land, the hafka'ah would be to put it in a situation where it is exempt from ma'aser (the answer of the gemara is that by a house the mitzvah only begins with a jewish person living in the house, meaning it is not a responsibility on the house, rather on the person living there, whereas by a field it is really an obligation on the field so selling to a goy qualifies as a hafka'ah).

Now, Rashi and Tosafos argue on the girsa of the mishna. The basis for their argument is that Rashi assumes that if m'doraysa the field of a goy is chayev in ma'aser, it is also chayev in bikurim, therefore even if the jew sold the filed to a goy, he would be responsible to buy back the bikurim to do the mitzvah (since it is incumbent upon him). Tosafos understands that even if the field owned by a goy is exempt, there would not be a chiyuv on the Jew who sold it to buy back the fruits to bring the bikurim (therefore the responsibility to buy it back would at most be a takanas chachamim). They seem to be arguing whether we conisder bikurim a mitzvah that is incumbent upon the person (and therefore he would have to make sure he fulfills his mitzvah even though the goy may charge a lot to purchase back the fruits), whereas Tosafos does not consider Bikurim to be incumbent on the Jew. The Maharsha draws a parallel between mezuza and bikurim. Just as mezuza is a mitzvah on the jewish owner, nevertheless he may sell the house to a goy because it is not considered a hafka'ah, so too bikurim is a mitzvah on the Jew but he need not buy back the fruits from the goy because although the goy won't give bikurim it isn't considered a hafka'ah since the land is not in the control of a jew to do it. The Maharsha is very strange, because he implies that when a jew sells a house to a goy, the jew remains technically liable to put up a mezuza just that the goy's ownership makes it unable for him to do so! See Karnei R'eim.