Ibn Kathir (Abridged)
Hafiz Ibn KathirFirst, he (Shu`ayb) prohibited them from cheating in business by decreasing the weights whenever they gave (products) to people. He commanded them to give just measure and weight whether they were giving or receiving (in transactions). He also forbade them from causing mischief and corruption in the land. This was due to their practice of highway robbery along the roads.
Abu Ja`far bin Jarir said, بَقِيَّتُ اللَّهِ خَيْرٌ لَّكُمْ(That which is left by Allah (after giving the rights of the people) is better for you,) "This means what you gain from your successful business dealings in which you have given just measure, is better for you than wrongfully taking the wealth of the people." Ibn Jarir said that this statement has been reported from Ibn `Abbas and I say it is similar to Allah's statement, قُل لاَّ يَسْتَوِى الْخَبِيثُ وَالطَّيِّبُ وَلَوْ أَعْجَبَكَ كَثْرَةُ الْخَبِيثِ(Say: "Not equal are Khabith (all that is evil) and Tayyib (all that is good), even though the abundance of Khabith may please you.")5:100 Allah then says, وَمَآ أَنَاْ عَلَيْكُمْ بِحَفِيظٍ(And I am not a guardian over you.) This means a watcher over you people. In other words, "Do this for Allah and not to be seen by the people."