Is it permissible for a man to marry the sister of his friend, while the other friend marries the sister of the first?

Shighar marriage has three forms:

1. Explicit Shighar: This is when one man says to another: "I marry you to my female ward on condition that you marry me to your female ward, and the private parts of each of them will be the mahr (dowry) for the other." This marriage is invalid according to all scholars. The Prophet, peace and blessings be upon him, forbade it, and they unanimously agreed on its prohibition. They differed, however, on its validity if it occurs. Some said it is invalid and must be annulled, while others said it is valid with sin, and the mahr al-mithl (customary dowry) is obligatory.

2. Implicit Shighar: This is when one says: "I marry you to my female ward for such-and-such (a mahr) on condition that you marry me to your female ward for such-and-such (a mahr)." This marriage is invalid according to the Malikis. It is annulled before consummation, and upheld after consummation with the higher of the specified amount or the customary dowry.

3. Compounded Shighar: This is when one says: "Marry me to your daughter for one hundred dinars, for example, on condition that I marry you to my daughter." This marriage is also invalid. The marriage of the woman for whom no mahr was specified is annulled. As for the woman for whom a mahr was specified, her marriage is annulled before consummation and upheld after consummation with the higher of the specified dowry or the customary dowry.

These forms should be avoided due to the harm they inflict upon women and the prioritization of the husbands' interests over those of the female wards. The اصل (fundamental principle) is that each woman should be contracted individually according to her own interest.