Surat An-Nahl (93-94)

{وَلَوْ شَاء اللّهُ لَجَعَلَكُمْ أُمَّةً وَاحِدَةً وَلكِن يُضِلُّ مَن يَشَاءُ وَيَهْدِي مَن يَشَاءُ وَلَتُسْأَلُنَّ عَمَّا كُنتُمْ تَعْمَلُونَ} (النحل: 93)

93- If Allah had willed to make you one nation holding the same religion, He would have done so, but He gives you the choice. He has shown you the path of guidance and error, so Allah would lead astray those who follow the way of evil and error and guides whomever He wills to the path of goodness and guidance. In both case, each one of them shall be reckoned for his actions whether good or bad.

{وَلاَ تَتَّخِذُواْ أَيْمَانَكُمْ دَخَلاً بَيْنَكُمْ فَتَزِلَّ قَدَمٌ بَعْدَ ثُبُوتِهَا وَتَذُوقُواْ الْسُّوءَ بِمَا صَدَدتُّمْ عَن سَبِيلِ اللّهِ وَلَكُمْ عَذَابٌ عَظِيمٌ} (النحل: 94)

94- Do not take your oaths as cheat and deceit when you deal with one another, causing a person to deviate from the truth after he had been firm on the truth. A person who does so shall be sinful for his oath because he caused people to detest from religion. If a Muslim makes an oath to a disbeliever then did not keep it, a disbeliever shall not trust him or his religion, therefore a Muslim shall be sinful; and whoever does so shall be punished severely.

Keeping to promises is a good manner which caused many people to embrace Islam because of the truthfulness of merchants and keeping to their promises.