In The Name of Allah, The Most Merciful, The Bestowed of Mercy.

Allah [The Exalted] said:

وَيْلٌ لِّلْمُطَفِّفِينَ
الَّذِينَ إِذَا اكْتَالُوا عَلَى النَّاسِ يَسْتَوْفُونَ
وَإِذَا كَالُوهُمْ أَو وَّزَنُوهُمْ يُخْسِرُونَ
أَلَا يَظُنُّ أُولَٰئِكَ أَنَّهُم مَّبْعُوثُونَ
لِيَوْمٍ عَظِيمٍ
يَوْمَ يَقُومُ النَّاسُ لِرَبِّ الْعَالَمِينَ

Woe to Al-Mutaffifin [those who give less in measure and weight (decrease the rights of others)], 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 men, give less than due. Think they not that they will be resurrected (for reckoning), on a Great Day, the Day when (all) mankind will stand before the Lord of the ‘Alamin (mankind, jinns and all that exists)? [Surah Al-Mutaffifeen. Aayaat 1-7]

[وَيْلٌ- Woe ] is a word of punishment and threat of divine vengeance to those who give less in measure. (1) Qaadhi Iyaad [may Allah have mercy upon him] said that the word ‘’Waylun‘’ (Woe) has some of the following meanings: It is used whilst referring to one who has fallen into destruction. It is used whilst referring to one who deserves destruction. It is used to refer to destruction itself. It is used to refer to the difficulty found in punishment. It is used to refer to grief. It is used to refer to a valley in the hell fire. (2)

Allah explained who the Mutaffifeen are in his statement: [الَّذِينَ إِذَا اكْتَالُوا عَلَى النَّاسِ يَسْتَوْفُونَ – Those who, when they have to receive by measure from people, demand full measure] -Meaning, they take from them in full what is established to be their due and demand in full without any loss.

[وَإِذَا كَالُوهُمْ أَو وَّزَنُوهُمْ يُخْسِرُونَ -And when they have to give by measure or weight to people, give less than what is due] -Meaning, when they give the people what they are entitled to by way of measure or weight, they give them less- either by giving short measure or weight, or by not giving the full measure or weight, or by not filling the measuring or weighing equipment and what is similar that. This is tantamount to stealing the people’s wealth and not being fair to them. If this is the threat of divine vengeance against those who give short measure and short weight, then those who take people’s wealth by force or by stealing are more deserving of this threat than those who give short measure and short weight.

This noble Aayah shows that just as a person takes from the people what he is entitled to, it is also obligated on him to give them everything they are entitled to – whether related to wealth or mutual dealings; rather included in the generality of this Aayah are proofs and statements, because indeed what usually takes place between those engaged in argumentation and discussion is that each of them is eager to establish his proofs, therefore, it is obligated on a person also to make known the proofs possessed by the other person and examine the proofs of the other person just as he examines his own proofs. In relation to this affair, the justice of a person is known as opposed to whether he is afflicted with bigotry, his humility as opposed to being haughty, his common sense as opposed to foolish behaviour. We ask Allah bestow on us every good.

Then Allah issued a threat to those who give short measure, stated with amazement regarding their state of affairs and their persistence upon what they are doing. Allah said:

أَلَا يَظُنُّ أُولَٰئِكَ أَنَّهُم مَّبْعُوثُونَ
لِيَوْمٍ عَظِيمٍ
يَوْمَ يَقُومُ النَّاسُ لِرَبِّ الْعَالَمِينَ

Think they not 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 – Meaning, what makes them dare to give short measure is because they do not believe in the Last Day, otherwise had they believed in it and known that they will stand before Allah to give account – whether related to something small or big, they would stopped and repent. (3)

[Ref 1: An Excerpt from Tafseer As-Sadi. Slightly paraphrased]
[Ref 2: Quoted by Shaikh Abdullaah Al-Bukhaari in Sharh Umdatul Ahkaam: lesson 2]
[Ref 3: An Excerpt from Tafseer As-Sadi. Slightly paraphrased]