The Prophet ﷺ 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.  

أَنَّ ابْنَ عُمَرَ ؓ كَانَ يَقُولُ نَهَى النَّبِيُّ ﷺ أَنْ يَبِيعَ بَعْضُكُمْ عَلَى بَيْعِ بَعْضٍ وَلاَ يَخْطُبَ الرَّجُلُ عَلَى خِطْبَةِ أَخِيهِ حَتَّى يَتْرُكَ الْخَاطِبُ قَبْلَهُ أَوْ يَأْذَنَ لَهُ الْخَاطِبُ  

"The Messenger of Allah forbade offering more for something that has already been bought by his brother, or for a man to propose marriage to a woman when someone else has already proposed to her, unless the previous suitor gave up the idea or gave him permission."  

أَنَّ عَبْدَ اللَّهِ بْنَ عُمَرَ كَانَ يَقُولُ نَهَى رَسُولُ اللَّهِ ﷺ أَنْ يَبِيعَ بَعْضُكُمْ عَلَى بَيْعِ بَعْضٍ وَلاَ يَخْطُبُ الرَّجُلُ عَلَى خِطْبَةِ الرَّجُلِ حَتَّى يَتْرُكَ الْخَاطِبُ قَبْلَهُ أَوْ يَأْذَنَ لَهُ الْخَاطِبُ  

The Prophet ﷺ 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. (Using translation from Bukhārī 5142)   

أَنَّ ابْنَ عُمَرَ ؓ كَانَ يَقُولُ نَهَى رَسُولُ اللهِ ﷺ أَنْ يَبِيعَ بَعْضُكُمْ عَلَى بَيْعِ بَعْضٍ وَلَا يَخْطُبُ الرَّجُلُ عَلَى خِطْبَةِ أَخِيهِ حَتَّى يَتْرُكَ الْخَاطِبُ قَبْلَهُ أَوْ يَأْذَنَ لَهُ الْخَاطِبُ