The Mishna says that slaves share the status of land regarding the exclusions listed. Therefore, a sale of a slave is not subject to the rules of ona’ah. Abaye says that a rental is subject to the rules of ona’ah, since it is equivalent to a temporary sale.
The Rishonim explain that Abaye is only referring to rental of items whose regular sale is subject to ona’ah, but rental of land is not subject to ona’ah, similar to a permanent sale of land.
The Rishonim discuss whether hiring a worker is subject to ona’ah. The Ramban and Rashba say that hiring a worker is not subject to ona’ah, since the Torah states that ona’ah applies when buying or selling a “mimkar” - a sale item. When hiring a worker, there is no sale item per se, and therefore no ona’ah.
The Rambam (Mechira 13:15, 17) says that hiring a worker is not subject to ona’ah, since it is akin to renting a slave. Since buying a slave is not subject to ona’ah, renting one – which is a temporary sale – is also not subject to ona’ah. However, the Rambam says that when hiring a worker for a project, as opposed to hourly work, ona’ah does apply, since such a transaction is not considered a temporary sale of a slave, but a proper transaction of merchandise.
The Drisha (227:47) explains that a slave is defined by his time being owned by his owner. Therefore, an hourly worker can be considered temporarily enslaved, since during his employment period, his time is owned by the employer, while a project worker is not even temporarily enslaved, since his time is always only his. Since the Rambam exempted employment as a function of a slave’s exclusion, project work, which is not similar to a slave’s work, is not exempted. However, the Ramban and Rashba offer a more fundamental reason to exempt employment from ona’ah, and therefore apply this to all types of employment, including project work.
This dispute among the Rishonim would seem to depend on a general dispute among the Rishonim about exclusions of slaves. Rashi (Kiddushin 7a, 28a) and Tosfos (Megilla 23b Shamin) say that whenever the Gemora makes halachic statements about slaves, this applies to any person, even if he is free. Therefore, the Gemora (Kiddushin 7a) considers a wife being betrothed to be equivalent to real estate (as far as modes of acquisition), and the Gemora (Kiddushin 28a) treats someone’s claim that one is his Jewish slave to be equivalent to a dispute over land (as far as swearing). Tosfos (Kiddushin 7a, 28a) and the Ritva (Kiddushin 28a), however, say that the categorization of slaves as equivalent to land only applies to Kena’ani slaves, and not to free people, or even to Jewish slaves. The Tur and Shulchan Aruch (HM 227:33,36) rule like the Rambam.
The Shach (HM 95:18) rules that the halachic rules of a slave apply to all people, since the Torah is simply using slaves as a vehicle to explain that human acquisition is equivalent to land acquisition. In general, only Kena’ani slaves are acquired, which is why the Torah used them to teach this rule.This is consistent with the position of the Shulchan Aruch.
The Kovetz Shiurim (Bava Basra 310) suggests that the Rambam may not rule that the laws of slaves apply to all people. However, this is because only a slave can be truly permanently acquired, while other situations (e.g., a wife or Jewish slave), are only temporary, and cannot be compared to land. However, in regard to ona’ah, the exclusion of a slave also excludes hourly employment. Abaye explained that ona’ah applies to rental, only since it is considered a temporary sale. Therefore, a rental is subject to ona’ah where an equivalent permanent sale is subject to ona’ah. Although the employee does not have the rules of a slave, and cannot be permanently bought, employment’s theoretical permanent counterpart would be enslavement, which is not subject to ona’ah. Therefore, the temporary sale of employment cannot be subject to ona’ah, since ona’ah derives from considering a rental as a temporary sale, as Abaye stated.