Caution: Translations of Quran and Ḥadīth may lead to possible misapplications and misinterpretations. This site is intended for students of sacred knowledge that are proficient in comprehending classical Arabic and have a strong foundation in Islamic sciences. Also note that religious injunctions rely on several aspects beyond what one may glean through reading individual aḥādīth.
ahmad:6417ʿAbd al-Ṣamad > Ṣakhr > Nāfiʿ > Ibn ʿUmar

[Machine] The Messenger of Allah ﷺ forbade selling a sale that has already been made and he used to say, "Do not overbid in transactions and do not sell what has already been bought, and do not propose to a woman who is already engaged to her brother until he leaves or gives permission to propose."  

أحمد:٦٤١٧حَدَّثَنَا عَبْدُ الصَّمَدِ حَدَّثَنَا صَخْرٌ عَنْ نَافِعٍ عَنِ ابْنِ عُمَرَ قَالَ

نَهَى رَسُولُ اللهِ ﷺ أَنْ يَبِيعَ حَاضِرٌ لِبَادٍ وَكَانَ يَقُولُ لَا تَلَقَّوْا الْبُيُوعَ وَلَا يَبِعْ بَعْضٌ عَلَى بَيْعِ بَعْضٍ وَلَا يَخْطُبْ أَحَدُكُمْ أَوْ أَحَدٌ عَلَى خِطْبَةِ أَخِيهِ حَتَّى يَتْرُكَ الْخَاطِبُ الْأَوَّلُ أَوْ يَأْذَنَ لَهُ فَيَخْطُبَ  


See similar narrations below:

Collected by Bukhārī
bukhari:5142Makkī b. Ibrāhīm > Ibn Jurayj > Nāfiʿ > Ibn

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.  

البخاري:٥١٤٢حَدَّثَنَا مَكِّيُّ بْنُ إِبْرَاهِيمَ حَدَّثَنَا ابْنُ جُرَيْجٍ قَالَ سَمِعْتُ نَافِعًا يُحَدِّثُ

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