(Background: After explaining why a wife nowadays no longer has the power to force her husband to move to Eretz Yisroel according to Rabbi Meir of Rothenberg, and explaining why a wife is different from a slave in this regard, the Rebbe quotes the Avnei Nezer, who in fact equates the wife with the slave and concludes that the wife does have power even today.)
The Avnei Nezer, Yoreh Deah siman 454, writes at first that the Beis Shmuel is alone in ruling in accordance with Rabbi Meir of Rothenberg; all other poskim hold that the wife’s power is in full force even today. His proof is that the law that a slave may force his master to free him when he wishes to go to Eretz Yisroel is codified in Yoreh Deah 267:84, where it is stated explicitly that it applies even today. Now, a master’s power over his slave is ten times stronger than a husband’s power over his wife, so we conclude that a wife must certainly have the power to force him to move to Eretz Yisroel. Also, the Taz and the Shach are silent and do not mention any distinction nowadays between husband and wife, so we see that they disagree with the Beis Shmuel.
Now, I have already explained at length the reasons why one cannot prove the law of a wife from the law of a slave. (The law of the slave is from the Torah while the law of the wife is Rabbinic (Siman 2), the slave has to support himself while the wife depends on her husband so she cannot force him to go to a place where he might not make a living (Siman 3) and we don’t want wives using this law as means to escape from marriage and marry another man (Siman 7).)
And even the Avnei Nezer himself, later in the same responsum, paragraph 22, offers a distinction between the slave and the wife. He proposes that nowadays, the mitzvah to live in Eretz Yisroel is only for righteous individuals. Therefore, the husband may claim that he meets this qualification and wants to go, and if his wife disputes this and refuses to go, she receives no kesubah payment because he can argue, “Prove to me that I am not righteous and then you will get your money.” As always, whichever party wishes to extract the money from the other has the burden of proof. But if the wife claims she is righteous and want to move, the husband can argue, “Prove to me that you are righteous and you will get your money.” Now, a slave is considered to be holding himself and so the master has burden of proof. If the slave claims to be righteous, he can say to his master, “Prove that I’m not righteous and going to Eretz Yisroel is not a mitzvah for me; otherwise I have the right to my freedom.”
However, what is difficult here is that the Avnei Nezer proposes to make this distinction according to the Rambam. The Rambam states clearly that the law of the slave applies even today, while when it comes to the husband and wife forcing one another, he does not say anything. And it’s unlikely that the Rambam did not bother to mention that the law still applies nowadays because he relied on us getting that information from his Laws of Slaves. So the Avnei Nezer understood that the Rambam held like Rabbi Meir of Rothenberg who says that the law of both spouses forcing does not apply nowadays, but rather only the husband can force the wife to move, and he gives the above explanation for the difference between a wife and a slave. The problem here is that the Rambam never mentions any distinction between husband and wife, and so it doesn’t seem that he held like Rabbi Meir of Rothenberg. If he held the law of forcing does not apply nowadays, then it does not apply at all – neither party can force the other to move. But that would be too radical, because it would place the Rambam in the same camp as Rabbeinu Chaim in Tosafos, who says that the law of forcing does not apply today since there is no mitzvah to live in Eretz Yisroel now, but the poskim say that Rabbeinu Chaim is a lone opinion. No one – not the Beis Yosef or anyone else – mentions that the Rambam agrees with him.
So it must be that all the poskim understood that the Rambam really means that the law of husband and wife forcing does apply even nowadays, and the only reason he does not say so is because he says so later on in the Laws of Slaves, and he relied on his readers deriving it from there. This is despite the fact that such reliance on later laws would run contrary to the principles the Rambam followed when writing his code.
The Yad Malachi in his “Rules about the Rambam” section 6 quotes the Knesset Hagedolah and the Tosafos Yom Tov (Orlah 5 and Parah 11), who say that the Rambam sometimes considers it sufficient to write something in one case early in his code and let his readers apply the rule to other cases later on in his code, but never does he leave out something in an early place and rely on the fact that he will write it in a different case later in his code. Then the Yad Malachi quotes a different place (Gittin 3) where the Tosafos Yom Tov seems to contradict this rule; he also cites a Kesef Mishneh in the Laws of Vows and other places where he seems to disagree with this rule. Therefore the Yad Malachi leaves the matter undecided.
In my humble opinion, the quote from Tosafos Yom Tov in Gittin does not prove anything, because he only writes that the Rambam in Laws of Divorce left something out and filled it in later in the Laws of Terumah because the Laws of Terumah is its natural place. So there is no comparison to our case, where the Laws of Slaves is no more a natural place for this law than the Laws of Marriage. The Yad Malachi’s quote from the Kesef Mishneh does not prove his point either, because he says that the law regarding oaths can be derived by kal vachomer (a fortiori) from that of vows. Thus in cases where there is no kal vachomer, the Rambam might not rely on writing someone in only one of the places. It is difficult to look up all the places quoted by the Yad Malachi, but it seems that unless there is a good reason to rely on something written later (like in these last two cases) the Rambam does not leave out things and rely on what he writes later. So it is hard to understand why he saw fit to tell us in the Laws of Slaves that the law applies nowadays, while in the Laws of Marriage – earlier in his code – he does not tell us this.
Possibly the poskim were uncertain as to the Rambam’s position on whether the law of forcing a spouse applies nowadays. Perhaps it does not apply, or perhaps it applies, and the Rambam had some good reason to leave it out and rely on what he writes in the case of the slave. Since the Rambam’s position was not known with certainty, no one lists him as agreeing with Rabbeinu Chaim. But this matter requires much further study.