﴿وَأَوْفُواْ الْكَيْلَ وَالْمِيزَانَ بِالْقِسْطِ﴾
(and give full measure and full weight with justice.) is a command to establish justice while giving and taking. Allah has also warned against abandoning this commandment, when He said,
﴿وَيْلٌ لِّلْمُطَفِّفِينَ - الَّذِينَ إِذَا اكْتَالُواْ عَلَى النَّاسِ يَسْتَوْفُونَ - وَإِذَا كَالُوهُمْ أَوْ وَّزَنُوهُمْ يُخْسِرُونَ - أَلا يَظُنُّ أُوْلَـئِكَ أَنَّهُمْ مَّبْعُوثُونَ - لِيَوْمٍ عَظِيمٍ - يَوْمَ يَقُومُ النَّاسُ لِرَبِّ الْعَـلَمِينَ ﴾
(Woe to Al-Mutaffifin. Those who, when they have to receive by measure from men, demand full measure. And when they have to give by measure or weight to (other) men, give less than due. Do they not think that they will be resurrected (for reckoning). On a Great Day The Day when (all) mankind will stand before the Lord of all that exists) ﴿83:1-6﴾. Allah destroyed an entire nation that was accustomed to giving less in weights and measures. Allah said next,
﴿لاَ نُكَلِّفُ نَفْسًا إِلاَّ وُسْعَهَا﴾
(We burden not any person, but that which he can bear.) that is, whoever strives while pursuing his rights and giving other peoples' full rights, then there is no sin on him if he commits an honest mistake after trying his best and striving to do what is right.