Marwan married his daughter to Abd al-Rahman bin Hakam, and Abd al-Rahman married his daughter to Marwan, and they had set a dowry for each. Muawiya bin Abu Sufyan, who was then the caliph, wrote to Marwan ordering him to separate the two couples. In his letter he said, "This is the shighar marriage that the Messenger of Allah ﷺ forbade."
أنه أنكح عبد الرحمن بن الحكم ابنته، وأنكحه عبد الرحمن ابنته، وقد كانا جعلا صداقا. فكتب معاوية بن أبي سفيان-وهو خليفة- إلى مروان يأمره بالتفريق بينهما. وقال في كتابه: هذا الشغار الذي نهى عنه رسول الله ﷺ.
Narrated by Abu Dawud (2075), and through his route by Bayhaqi (2007), from Muhammad bin Yahya bin Faris, who said, Yaqub bin Ibrahim narrated to us, who said, my father narrated to us, from Ibn Ishaq, who said, Abdul-Rahman bin Hurmuz Araj narrated to me, that Abbas bin Abdullah bin Abbas married his daughter to Abdul-Rahman bin Hakam, and he related the rest of the story.Ahmad (16856) and Ibn Hibban (4153) also narrated it, all of them through the route of Yaqub bin Ibrahim, with the same wording.Its chain of transmission is hasan because of Ibn Ishaq, who is known to conceal parts of his chains of transmission, except that here he stated his source explicitly, so the suspicion of concealment was removed.As for his statement, "They had made it a dowry," it means they turned the reciprocal marriage exchange into one with a dowry. But if a man marries off his daughter, and the other man marries his own daughter to him in return along with a dowry, then this takes it out of the prohibited reciprocal exchange marriage without a dowry.