I really wanted to write about this yesterday when it was fresh in my mind but I have other things to do beside blogging.
At any rate as fast as I can let me go over the major points.
In Yevamot (Tractate Yevamot) we find that Rav Awiya says a Jew that buys a gentile from another gentile the fellow jumps in the mikvah (body of natural water) and becomes free because the first gentile only had possession of the work of the hands. But if the gentile sold himself, [in a case let's say when he needs to pay off his debts] then he can't do this.
This is how Reb Joseph Karo understands the Rambam. The Rambam says a gentile buys a gentile from a different gentile he can do the jumping into the mikvak trick and he is free. It seems clear that from the way the Rambam puts it, that if he sold himself he can't do this trick.
Later when the Rambam says even acquiring the gentile from another gentile, the Jew gets possession of the body simply means after the mikvah was done for the sake of slavery.
So far the Rambam looks perfectly clear. The only thing Reb Chaim has against this are several points which are easily answered.
[I really would not say anything but in spite of the greatness of Reb Chaim opening up the whole idea of learning the Rambam in a deep way I still have found too many flaws] In the thing I wrote about before about work done not for its sake also everything the Reb Chaim said was great except that it contradicted the Rambam about the definition of a work that is not intended.
Reb Chaim is depending on an "even though" (Laws of Slavery chapter 9). Even though the Jews only buys the work of the hands, he still acquires the body. Reb Chaim is wondering what this even though is doing here. I answer: Because you might say the normal way of acquiring a slave is when he sells himself. So here we have something new.
However just to say a word on the side of Reb Chaim; he is understanding that Rav Achai disagrees with Rav Awiya about the case the Jew acquires the gentile from himself that there also the gentile can jump in the mikvah. It is not at all clear that this is what Rav Achei means.