"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  

A man must not make proposal of marriage to a woman when his brother has done so already. And he must not offer a price for a thing for which his brother had already offered a price; and a woman must not be combined in marriage with her father's sister, nor with her mother's sister, and a woman must not ask to have her sister divorced in order to deprive her of what belongs to her, but she must marry, because she will have what Allah has decreed for her. (Using translation from Muslim 1408g)  

عَنِ النَّبِيِّ ﷺ قَالَ لَا يَسُومُ الرَّجُلُ عَلَىسَوْمِ أَخِيهِ وَلَا يَخْطُبْ عَلَى خِطْبَةِ أَخِيهِ