Is This a Form of Ash-Shighar?
Question :
I wish to marry the daughter of one of my kin according to the Sunnah of Allah and His Messenger . He has a son whom I wish to marry to my sister, according to the Sunnah of Allah and His Messenger. Is this permissible, or not, bearing in mind that the dowries will not be equal, and the individual rights of the two women will not be the same. They are both agreeable to this, and neither of them objects to it?
Answer:
If the situation is as you have described, that the two girls. agree, and that a dowry will be paid to each of them before, without any deception, and that there exists no condition between you whether spoken or unspoken - which necessitates that he will marry you to his daughter on the understanding that you will marry his son to your sister, then there is no objection to that. There is nothing to prevent it in the Islamic Law. And Allah is the Granter of success. And may peace and blessings be upon our Prophet, Muhammad and upon his family and Companions.
Source:
The Permanent Committee
Fatawa Islamiyah, Vol. 5 Pages 420-421