وَيْلٌۭ لِّلْمُطَفِّفِينَ ﴿١﴾
伤哉！称量不公的人们。
Haram Money & business
Is cheating in business haram?
Short answer
Yes. The Quran warns: woe to those who give less than is due, and commands full measure and weight (83:1-3, 11:85). The Prophet ﷺ said: he who deceives is not of me.
Evidence
Quran
ٱلَّذِينَ إِذَا ٱكْتَالُوا۟ عَلَى ٱلنَّاسِ يَسْتَوْفُونَ ﴿٢﴾
当他们从别人称量进来的时候，他们称量得很充足；
وَإِذَا كَالُوهُمْ أَو وَّزَنُوهُمْ يُخْسِرُونَ ﴿٣﴾
当他们量给别人或称给别人的时候，他们不称足不量足。
وَيَٰقَوْمِ أَوْفُوا۟ ٱلْمِكْيَالَ وَٱلْمِيزَانَ بِٱلْقِسْطِ ۖ وَلَا تَبْخَسُوا۟ ٱلنَّاسَ أَشْيَآءَهُمْ وَلَا تَعْثَوْا۟ فِى ٱلْأَرْضِ مُفْسِدِينَ ﴿٨٥﴾
我的宗族啊！你们应当使用充足的斗和公平的秤，你们不要克扣他人应得的财物，不要在地方上为非作歹。
Hadith
Sahih Muslim 102
It is narrated on the authority of Abu Huraira that the Messenger of Allah (ﷺ) happened to pass by a heap of eatables (corn). He thrust his hand in that (heap) and his fingers were moistened. He said to the owner of that heap of eatables (corn):What is this? He replied: Messenger of Allah, these have been drenched by rainfall. He (the Holy Prophet) remarked: Why did you not place this (the drenched part of the heap) over other eatables so that the people could see it? He who deceives is not of me (is not my follower)
Arabic text (with chain of narrators)
وَحَدَّثَنِي يَحْيَى بْنُ أَيُّوبَ، وَقُتَيْبَةُ، وَابْنُ، حُجْرٍ جَمِيعًا عَنْ إِسْمَاعِيلَ بْنِ جَعْفَرٍ، - قَالَ ابْنُ أَيُّوبَ حَدَّثَنَا إِسْمَاعِيلُ، - قَالَ أَخْبَرَنِي الْعَلاَءُ، عَنْ أَبِيهِ، عَنْ أَبِي هُرَيْرَةَ، . أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم مَرَّ عَلَى صُبْرَةِ طَعَامٍ فَأَدْخَلَ يَدَهُ فِيهَا فَنَالَتْ أَصَابِعُهُ بَلَلاً فَقَالَ " مَا هَذَا يَا صَاحِبَ الطَّعَامِ " . قَالَ أَصَابَتْهُ السَّمَاءُ يَا رَسُولَ اللَّهِ . قَالَ " أَفَلاَ جَعَلْتَهُ فَوْقَ الطَّعَامِ كَىْ يَرَاهُ النَّاسُ مَنْ غَشَّ فَلَيْسَ مِنِّي " .
This is general information, not a fatwa. For your own situation, ask a qualified scholar.
Questions about halal and haram
Is cheating in business haram?
Yes. The Quran warns: woe to those who give less than is due, and commands full measure and weight (83:1-3, 11:85). The Prophet ﷺ said: he who deceives is not of me.