One Narration/Hadith and different Additions
First of all the Hadith itself ended with "Allah's Apostle forbade the Shighar." the rest of it is more or less ijtihad as it's based on a saying of the tabi'i Nafi' mawla Abdullah ibn 'Omar (May Allah be pleased with them).
And after that came some fiqh terms such as "the marriage is valid but its condition is illegal", which again have been added to the hadith and the statement of Nafi'. As i will show those people who used these terms came after Nafi' so this addition might have been made by al-Bukhari himself!
This Narration explains that our Messenger (peace be upon him) forbade to marry according to shighar الشِّغَارُ which is a marriage Nafi' explained, one could say it's a kind of marriage "in exchange": I marry your daughter and you mine but we don't give any mahr, as Imam Nafi' explained in his saying!
Nikaah al-shighaar refers to when a man gives his daughter, sister or other female relative whose guardian he is in marriage on the basis that the other man will give him a female relative in marriage, or will let his son or nephew marry his daughter, sister or niece, and so on. The marriage contract done on this basis is invalid, whether a mahr is stated or not (Fatwa)
Note: By relative or female relative of a man I mean a woman for whom this man is considered as a legal guardian/waly like his daughter, etc.
- "Some people" refers as most scholar say to Abu Hanifa!
- "Some others" refers as most scholars say to some Scholars (Zofar according to ibn Hajar) of the hanafi madhab (not all of them!)!
These later quotes are from al-Bukhari himself!
What does it man by the marriage is valid but its condition is illegal?
First you should be aware that this statement is for a marriage by a trick!
- The condition in Shighar: is that man A only allows man B to marry his daughter or relative if man B "in exchange" gives him a female relative for marriage.
So in this kind of marriage (even if there is a mahr/sadaq defined) the condition gives it a taste as if the real mahr for the first marriage is that man A can marry a relative of man B. Note that the majority (some details see below) of scholars are in consensus that once there's a predefined mahr this marriage is technically valid. So the major issue with shighar is the absence of a mahr!
- The condition in Mut'a: is the time: The marriage will end after a given time!
Abu Hanifa said about shighar: the marriage is valid but the condition is illegal: This means he would accept this marriage. In the hanafi madhab if there's no mahr given in that case it would be considered as equal mahr (or sadaq) صداق المثل for both marriages.
There's a rule in the Hanafi madhab saying:
ما لم يشرع بأصله ووصفه باطل وما شرع بأصله دون وصفه فاسد
(My own translation, so take it with care)
What is not prescribed by it's basis أصل and it's attributes وصف is invalid باطل and what is prescribed by it's source but without it's detail is illegal فاسد.
- In this case marriage (nikah) is a well known and given and well described matter in shari'a it's the basis (origin) أصل here but the condition of shighar (which is the attribute وصف) is not an accepted form of sadaq (mahr) therefore: the condition is illegal! This is like marrying without sadaq (mahr)!
- For muta' abu Hanifa said that the marriage is illegal (as muta' is considered as illegal) and invalid according to the rule above.
- According to al Qustalani quoting al-'Ayni the opinion of Zofar (a student of abu Hanifa) about muta' is as follows: if a man married a woman for a certain time the marriage itself is valid but the condition (a given time) is illegal.
- According to this fatwa (unfortunately only in Arabic) in case of a shighar like marriage (man A marries a relative of man B and vice versa, but without making it a condition for any of the marriages to be accepted) with mahr is legal marriage in all madhabs. So the problem in shighar was only the condition that there's no mahr but one could marry such a "marriage in exchange" if mahr was given (to the brides) on both sides!