Strengthening the Ummah

Nafi narrated to me that 'Abdullah said that Allah's Messenger (ﷺ) forbade
the Shighar. I asked Nafi', "What is the Shighar?" He said, "It is to
marry the daughter of a man and marry one's daughter to that man (at
the same time) without Mahr (in both cases); or to marry the sister of
a man and marry one's own sister to that man without Mahr." Some
people said, "If one, by a trick, marries on the basis of Shighar, the
marriage is valid but its condition is illegal." The same scholar said
regarding Al-Mut'a, "The marriage is invalid and its condition is
illegal." Some others said, "The Mut'a and the Shighar are permissible
but the condition is illegal."