Siman 127

A man who borrowed from his wife, even with a shetar [Shach], and even if he divorced her; or a master from his slave, even with a shetar, and even if he emancipated him—they have nothing against him. This is so even if the husband or the master did not claim, "It is clear to me that the money is mine," because whatever a slave acquires, his master acquires, and whatever is in the hand of the wife is in the presumptive possession of her husband until she brings proof that this is from the money of her nedunyah. Since it is not known that they have money over which he has no authority—such as if it was given to them on the condition that the husband or the master has no authority over it—everything is in his authority. And even though when he sold her land, it was explained in Even HaEzer siman 85 that specifically when the money was hidden we say he intended to reveal the money, but regarding money that was not hidden his sale is a sale; nevertheless, regarding a loan, even with money that was not hidden, we say that his intention was to extract the money from her, for certainly he does not desire to be "a slave who borrows to a man who lends" [Sma]. Furthermore, since in a loan he removed the money entirely into his authority, we establish it in his presumptive possession. And some say that regarding a loan as well, there is a distinction between hidden and non-hidden money, for with non-hidden money we establish the money in their authority when they claim they have money over which the owners have no authority. And if she conducts business within the house, we always establish it in the authority of the husband until she brings proof, as I wrote in siman 62.

One who borrowed from a ger [in ancient times] whose sons converted with him and he died, or if the ger deposited a deposit with him and he died, he is not required to return it to his sons, for a ger who converted is like a newborn child and his sons do not inherit from him when he did not beget them in his state of conversion. And if he returned it to his sons, it is as if he returned it to others, and the sages do not hold him in high regard for this. In what case does this apply? When their conception and their birth were not in holiness. But if their birth was in holiness, even though their conception was not in holiness, and according to the law he is not required to return it to them, nevertheless, if he acted beyond the letter of the law and returned it to them, he has performed the will of the sages and their spirit is pleased with him. And this is specifically when his mother was also a gentile and converted before she gave birth to him; but if his mother is a Jewess and she became pregnant from a gentile, he is not related at all after his father, for the offspring is like her. And so too regarding a Jew who comes upon a gentile woman, the offspring is like her, as written in Even HaEzer siman 8. And there are some of our teachers who hold that in a case of their conception and their birth not being in holiness, the spirit of the sages is pleased with him when he acts beyond the letter of the law and returns it to them; and in a case of their conception not being in holiness but their birth being in holiness, the sages did not want him to return it to them so that they should not err to say that they have kinship and come to a stumbling block, for if he betroths his father's wife they will say that kiddushin does not take hold with her, and he will come to perform yibbum with his brother's wife and they will exempt her from chalitzah [Tosafot Kiddushin 18a].