If he marries his female ward to another man on the condition that the other man marries his own female ward to him, and they do so without any dowry, both marriages are invalid. ………
His statement: “If he marries his female ward to another man on the condition that the other man marries his own female ward to him, and they do so without any dowry, both marriages are invalid.” This is among invalid conditions which invalidate the contract. “If he marries his female ward”—that is, one over whom he has guardianship. Here, walīyah means one who is under guardianship; in other words, his female ward. “On the condition that the other man marries his female ward to him”—meaning, one over whom the other has guardianship, such as his daughter, sister, or paternal aunt. As for a grandmother: if she is his maternal grandmother, he has no guardianship over her; but if she is his paternal grandmother, then he is her grandson’s son, and thus has guardianship over her.
His words, “on the condition,” mean: on the condition that the other man marries his female ward to him, with no dowry between them. The dowry is the bridal gift due through the marriage contract. “Both marriages are invalid”: each of them is invalid. The evidence for this is both textual and rational. As for the textual evidence, it is established in al-Bukhari and Muslim, from the hadith of Ibn Umar, that the Prophet ﷺ forbade shighār marriage. Shighār was explained as a man marrying his female ward to another on the condition that the latter marries his female ward to him, with no dowry between them (1).
The rationale consists of three points:
First: in this case, he has made the sexual availability of each woman the dowry for the other. So, did he seek marriage through his wealth, or through the sexual availability of his female ward? The answer is that he sought it through the sexual availability of his female ward. Allah, Most High, says: “Lawful to you are all beyond these, provided you seek them in marriage with your wealth” [al-Nisa 4:24]—meaning, that you seek marriage through your wealth. Yet this man sought marriage through the sexual availability of his female ward, making her sexual availability the dowry.