Mishkat al-Masabih 11 Business Transactions كتاب البيوع 88

وَعَنْهُ قَالَ قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: «لَا يَبِعِ الرَّجُلُ عَلَى بَيْعِ أَخِيهِ وَلَا يَخْطِبْ عَلَى خِطْبَةِ أَخِيهِ إِلَّا أنْ يأذَنَ لَهُ» . رَوَاهُ مُسلم

He reported God’s Messenger as saying, “One must not offer more than the amount agreed by his brother*, or ask a woman in marriage when his brother has done so, unless he permits him.” *i.e. when the parties have already agreed on a price. Muslim transmitted it.