The commentaries discuss a similar difficulty in the Gemara. Rav Sheshes is quoted as clearly stating that the Eved cannot be redeemed halfway, while the Rambam rules (Hilchos Avadim 2:7) that he can be redeemed halfway. The simple explanation which is alluded to by the Kesef Mishneh (ibid.) is that the Rambam Paskens like Abaye, who says immediately after Rav Sheshes that "Im Timtza Lomar Nigal le'Chatza'in" -- "If you will say that he is redeemed halfway," implying that he argues on Rav Sheshes. However, the Kesef Mishneh himself is unsatisfied with this answer, as Abaye never clearly stated that he holds Nigal le'Chatza'in.

Rav Elyashiv Shlita, in He'aros, answers that there is an argument earlier (15b) between Rebbi and Rebbi Yosi ha'Gelili whether or not a Nimkar l'Nochri always goes out within six years. Rebbi says that he does not, which implies that he sets the amount of the time of the sale to whatever he wants, even one year. This implies that Rebbi does not agree with Rav Sheshes, as Rav Sheshes derives from a Pasuk (source of the Pasuk is unclear, see 18a) that all of him is sold, not half of him. Being that the Rambam Paskens like Rebbi in the previous argument, he cannot Pasken like Rav Sheshes. This overruling factor of Paskening like Rebbi would seemingly push away Ravina's answer as well, as Ravina is merely coming to defend a Tzad ha'Shaveh which defends a Beraisa by Batei Arei Chomah. If we Pasken like Rebbi, and therefore unlike Rav Sheshes, it is likely that the Rambam had another way to defend the Beraisa, or he simply held that the Tana of the Beraisa is a Tana who argues on Rebbi.

All the best,

Yaakov Montrose