This is why fiat is haram:


Surah Ash-Shu'ara, 26:سورة الشعراء
أَوْفُوا الْكَيْلَ وَلَا تَكُونُوا مِنَ الْمُخْسِرِينَ
(Give full measure and do not be of those who cause loss.)
​182. وَزِنُوا بِالْقِسْطَاسِ الْمُسْتَقِيمِ
(And weigh with the correct scales.
​183. وَلَا تَبْخَسُوا النَّاسَ أَشْيَاءَهُمْ وَلَا تَعْثَوْا فِي الْأَرْضِ مُفْسِدِينَ
(And do not deprive people of their due and do not commit abuse on the earth, spreading corruption.