1- Narrated '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 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."

http://en.wikipedia.org/wiki/Mahr

http://www.merriam-webster.com/netdict/dower

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Note added at 44 mins (2010-05-06 10:25:03 GMT)
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you can also use "dowry" instead of dower.

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Note added at 49 mins (2010-05-06 10:30:08 GMT)
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This means that a man gives his daughter or sister in marriage to another man in exchange for the other's daughter or sister without fixing / offering any dowry (mahr).

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Note added at 4 days (2010-05-10 15:24:52 GMT) Post-grading
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You are very welcome.