A believer is the brother of a believer, so it is not lawful for a believer to outbid his brother, and he should not propose an engagement when his brother has thus proposed until he gives it up.
أَنَّهُ سَمِعَ عُقْبَةَ بْنَ عَامِرٍ عَلَى الْمِنْبَرِ يَقُولُ إِنَّ رَسُولَ اللَّهِ ﷺ قَالَ الْمُؤْمِنُ أَخُو الْمُؤْمِنِ فَلاَ يَحِلُّ لِلْمُؤْمِنِ أَنْ يَبْتَاعَ عَلَى بَيْعِ أَخِيهِ وَلاَ يَخْطُبَ عَلَى خِطْبَةِ أَخِيهِ حَتَّى يَذَرَ