7. It is permissible to say to a worker (1) on Shabbos: "Do you think you will be able to join me this evening?" Although the worker infers from this statement that he needs to hire him that night (after Shabbos) for a specific task, [this is permissible,] since only explicit statements are forbidden.
One may not, however, tell a worker, "Be available for me tonight." This is considered equivalent to an explicit statement that one desires to hire him (2).
(1) Either Jew or Gentile.
(2) We saw in HY 90:4, that it is prohibited on Shabbos to talk about doing something after Shabbos, if that activity would be prohibited on Shabbos. Therefore, since it is prohibited to hire a worker on Shabbos itself, it is also prohibited on Shabbos, to explicitly talk about hiring him. However, we saw in HY 90:6 that thinking about business affairs on Shabbos is permitted (although we are advised not to do so). Hinting to a worker on Shabbos (as in the first case in today's halacha) is considered the equivalent to thinking about one's affairs, and is therefore permitted (See Shulchan Aruch 307:7, and the Mishna Berura there).