The Command to fulfill the Covenant
Allah says;
وَأَوْفُواْ بِعَهْدِ اللّهِ إِذَا عَاهَدتُّمْ
And fulfill the covenants (taken in the Name of) Allah when you have taken them,
This is one of the commands of Allah, to fulfill covenants, keep promises and to fulfill oaths after confirming them.
Thus Allah says;
وَلَا تَنقُضُواْ الَايْمَانَ بَعْدَ تَوْكِيدِهَا
and do not break the oaths after you have confirmed them.
There is no conflict between this and the Ayat;
وَلَا تَجْعَلُواْ اللَّهَ عُرْضَةً لاًّيْمَـنِكُمْ
And do not use Allah as an excuse in your oaths. (2;224)
ذلِكَ كَفَّارَةُ أَيْمَـنِكُمْ إِذَا حَلَفْتُمْ وَاحْفَظُواْ أَيْمَـنَكُمْ
That is the expiation for oaths when you have sworn. And protect your oaths. (5;89)
meaning, do not forgo your oaths without offering the penance.
There is also no conflict between this Ayah (16;91) and the Hadith reported in the Two Sahihs according to which the Prophet said;
إِنِّي وَاللهِ إِنْ شَاءَ اللهُ لَا أَحْلِفُ عَلَى يَمِينٍ فَأَرَى غَيْرَهَا خَيْرًا مِنْهَا إِلاَّ أَتَيْتُ الَّذِي هُوَ خَيْرٌ وَتَحَلَّلْتُهَا وَفِي رِوَايَةٍ وَكَفَّرْتُ عَنْ يَمِينِي
By Allah, if Allah wills, I will not swear an oath and then realize that something else is better, but I do that which is better and find a way to free myself from the oath.
According to another report he said;
"and I offer penance for my oath."
There is no contradiction at all between all of these texts and the Ayah under discussion here, which is;
وَلَا تَنقُضُواْ الَايْمَانَ بَعْدَ تَوْكِيدِهَا
(and do not break the oaths after you have confirmed them), because these are the kinds of oaths that have to do with covenants and promises, not the kind that have to do with urging oneself to do something or preventing him from doing something.
Therefore Mujahid said concerning this Ayah;
وَلَا تَنقُضُواْ الَايْمَانَ بَعْدَ تَوْكِيدِهَا
(and do not break the oaths after you have confirmed them),
"The oath here refers to oaths made during Jahiliyyah."
This supports the Hadith recorded by Imam Ahmad from Jubayr bin Mut`im, who said that the Messenger of Allah said;
لَاا حِلْفَ فِي الاْاِسْلَامِ وَأَيُّمَا حِلْفٍ كَانَ فِي الْجَاهِلِيَّةِ فَإِنَّهُ لَاا يَزِيدُهُ الاْاِسْلَامُ إِلاَّ شِدَّة
There is no oath in Islam, and any oath made during the Jahiliyyah is only reinforced by Islam.
This was also reported by Muslim.
The meaning is that Islam does not need oaths as they were used by the people of the Jahiliyyah; adherence to Islam is sufficient to do away with any need for what they used to customarily give oaths for.
In the Two Sahihs it was reported that Anas said;
"The Messenger of Allah swore the treaty of allegiance between the Muhajirin (emigrants) and the Ansar (helpers) in our house."
This means that he established brotherhood between them, and they used to inherit from one another, until Allah abrogated that. And Allah knows best.
وَقَدْ جَعَلْتُمُ اللّهَ عَلَيْكُمْ كَفِيلً
- and indeed you have appointed Allah as your guarantor.
إِنَّ اللّهَ يَعْلَمُ مَا تَفْعَلُونَ
Verily, Allah knows what you do.
This is a warning and a threat to those who break their oaths after confirming them.
Allah says