Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.
If the girl has expressed her consent to the proposal of marriage with a man then it is forbidden for any other man to propose to marry her unless she refused the proposal of that man in the first place or that man himself permits others to propose marrying her. The Prophet (Sallallahu Alaihi wa Sallam) said: “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.” [Al Bukhari]
Imam al-Sana-ani said in Subul al-salaam: ‘The consensus of Muslim scholars upheld the prohibition of proposing to a woman after she had accepted a proposal or her guardian had accepted a proposal if she is still young’.
Some scholars are of the opinion that it is permissible to ask for the hand of an already engaged girl if the first proposing man is dissolute.
Allah knows best.