Shevi'is, Chapter 4, Mishnah 3. This mishnah deals with the relationship between Jews and gentiles during the Shemittah year: in business dealings with them, in offering them a blessing, and in the question of greeting them.
"Chochrin nirin min ha'goyim ba'shevi'is":
A person is permitted to take a field from a gentile as a tenant farmer. Chakirus is a form of tenancy in which the tenant does not give the owner of the field a percentage of the produce, but rather a fixed sum. This arrangement is permitted with "nirin" - plowed fields - during the Shemittah year, in order to sow them at the conclusion of the seventh year, in the year following Shemittah, even though it causes the gentiles to plow the fields during Shemittah. Since they are not Jews, they are permitted to plow during Shemittah, and even though a Jew is the cause of their doing so, there is no prohibition in this.
Such an arrangement, however, may not be made with a fellow Jew, since by doing so one causes him to plow the field on his behalf during Shemittah, so that he will have a well plowed field at the conclusion of the seventh year.
"Machzikin yedei goyim ba'shevi'is, aval lo shel Yisrael":
One may strengthen the hands of gentiles who are working during Shemittah, but one may not strengthen the hands of a Jew. And what is meant by strengthening the hands? As the Rav explains: to wish him "yishar kochacha" - the blessing we commonly call "yasher koach," whose plain meaning is to straighten his strength, that it should be upright. One is permitted to offer this blessing to gentiles working during Shemittah, since from their standpoint there is no prohibition at all; but one should not offer it to a Jew, since he is committing a transgression and violating the Torah.
"U'sho'alin bishlomam mipnei darchei shalom":
Since one is permitted to offer a blessing to gentiles, one is also permitted to greet them and inquire after their welfare, and this is because of the ways of peace with our non Jewish neighbors.
The Gemara in Maseches Gittin explains this line in the mishnah: it is obvious that one may greet them - if one may bless them for their work during Shemittah, all the more so may one greet them. Rather, the mishnah comes to teach that one may greet them even on the days of their festivals, despite the concern that this might lead them to give thanks to their gods. Chazal ruled that notwithstanding this concern, one may inquire after their welfare and their wellbeing even on their festivals.
In summary: in this mishnah we learned that one may lease plowed fields from gentiles during Shemittah in order to sow them at the conclusion of the seventh year, even though this causes them to plow during Shemittah, whereas such an arrangement may not be made with a fellow Jew; that one may strengthen the hands of gentiles working during Shemittah with the blessing "yishar kochacha," but not the hands of a Jew who is transgressing; and that one may greet them because of the ways of peace, even on the days of their festivals.