"Let one of you not undersell another."[1]  

أَنَّ رَسُولَ اللَّهِ ﷺ قَالَ لاَ يَبِيعُ بَعْضُكُمْ عَلَى بَيْعِ بَعْضٍ  

"A man is not to undersell his brother, nor is he to fry to outhaggle his brother."  

عَنِ النَّبِيِّ ﷺ قَالَ لاَ يَبِيعُ الرَّجُلُ عَلَى بَيْعِ أَخِيهِ وَلاَ يَسُومُ عَلَى سَوْمِ أَخِيهِ