Hadiths 7.73 to 7.75

7.73:
Narrated Ibn `Umar: The Prophet 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.

7.74:
Narrated Abu Huraira: The Prophet said, "Beware of suspicion
(about others), as suspicion is the falsest talk, and do not
spy upon each other, and do not listen to the evil talk of the
people about others' affairs, and do not have enmity with one
another, but be brothers. And none should ask for the hand of
a girl who is already engaged to his (Muslim) brother, but one
should wait till the first suitor marries her or leaves her."

7.75:
Narrated `Abdullah bin `Umar: "When Hafsa became a widow,"
`Umar said, "I met Abu Bakr and said to him, 'If you wish I
will marry Hafsa bint `Umar to you.' I waited for a few days
then Allah's Apostle asked for her hand. Later Abu Bakr met me
and said, 'Nothing stopped me from returning to you concerning
your offer except that I knew that Allah's Apostle had
mentioned (his wish to marry) her, and I could never let out
the secret of Allah's Apostle . If he had left her, I would
have accepted her.' "