Neither of them possesses the right to dispose of what is in the possession of the other. The master’s right over what is in his [the mukatab's] possession is only attached due to the contingency that he might become incapable, in which case it would revert to him. This does not prevent the occurrence of usury between them, just as it is with a father and his son. According to this view, it is not permissible for there to be a disparity between them in that in which disparity is prohibited between two strangers, nor is deferment permissible in that in which deferment is prohibited between strangers.

Section: If each of them has a debt upon the other, such as if the master has a debt upon the mukatab from the kitabah or otherwise, and the mukatab has a debt upon his master, and both are cash of the same type, both immediate or both deferred to the same time, they settle the accounts (taqassa) and cancel each other out; for if they cancel each other out between strangers, it is even more appropriate between the master and his mukatab. If they are cash of two different types, such as dirhams and dinars, Ibn Abi Musa said: If he has one thousand dirhams upon his master, and his master has one hundred dinars upon him, and he designates them as a mutual set-off, it is permissible, unlike with two free men. Al-Qadi said: This is not permissible, because it is the sale of a debt for a debt, and the Prophet (peace and blessings of Allah be upon him) forbade the sale of debt for debt. And because it is not permissible between two strangers, it is not permissible between the mukatab and his master, like other prohibited things. It is distinct from the slave who is property (qinn); for he remains in the disposal of his master, and what is in his hand is the pure property of his master, who has the right to take it and dispose of it. According to this view, it is not permissible even with mutual consent. And according to the view of Ibn Abi Musa: It is permissible if they both agree to it and trade it, and the settlement (taqass) does not take effect before their mutual agreement, because it is a sale. As for if they are commodities ('ard), or a commodity and cash, it...