It was narrated that Ibn 'Umar said: "The Messenger of Allah P.B.U.H forbade Shighar. Shightir is when a man says to another man: 'Marry your daughter or sister to me, on condition that I will marry my daughter or sister to you: and they do not give any dower (i.e., neither of them give the other the dower)." (Sahih)