Ibn ‘Umar said “The Apostle of Allaah ﷺ prohibited shighar marriage. Musaddad added in his version “I said to ‘Nafi “What is shighar?” (It means that) a man marries the daughter of another man and gives his own daughter to him in marriage without fixing dower; and a man marries the sister of another man and gives him his sister in marriage without fixing dower.  

أَنَّ رَسُولَ اللَّهِ ﷺ نَهَى عَنِ الشِّغَارِ زَادَ مُسَدَّدٌ فِي حَدِيثِهِ قُلْتُ لِنَافِعٍ مَا الشِّغَارُ قَالَ يَنْكِحُ ابْنَةَ الرَّجُلِ وَيُنْكِحُهُ ابْنَتَهُ بِغَيْرِ صَدَاقٍ وَيَنْكِحُ أُخْتَ الرَّجُلِ وَيُنْكِحُهُ أُخْتَهُ بِغَيْرِ صَدَاقٍ  

Al-Abbas ibn Abdullah ibn al-Abbas married his daughter to Abdur Rahman ibn al-Hakam, and AbdurRahman married his daughter to him. And they made this (exchange) their dower. Mu'awiyah wrote to Marwan commanding him to separate them. He wrote in his letter: This is the shighar which the Messenger of Allah ﷺ has forbidden.  

وَقَالَ فِي كِتَابِهِ هَذَا الشِّغَارُ الَّذِي نَهَى عَنْهُ رَسُولُ اللَّهِ ﷺ