Bismillah Walhamdulillah Was Salaatu Was Salaam ‘ala Rasulillah
Narrated Ibn ‘Umar (Radi-Allahu ‘anhu):
The Prophet (Sallallahu ‘Alaihi Wa Sallam) decreed that one should not try to cancel a bargain already agreed upon between some other persons (by offering a bigger price). And a man should not ask for the hand of a girl who is already engaged to his Muslim brother, unless the first suitor gives her up, or allows him to ask for her hand.
Bukhari Vol. 7 : No. 73