What does it mean by the marriage is valid but its condition is illegal?

> **Sahih Bukhari Volume 009, Book 086, Hadith Number 090**. Narrated By > 'Abdullah : Nafi narrated to me that 'Abdullah said that Allah's > Apostle 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**." For a marriage to be legal all its conditions must be met.
Can a marriage be valid even if its conditions are illegal? **What does it man by the marriage is valid but its condition is illegal?**