And do not be like she who untwisted her spun thread after it was strong [by] taking your oaths as [means of] deceit between you because one community is more plentiful [in number or wealth] than another community. Allāh only tries you thereby. And He will surely make clear to you on the Day of Resurrection that over which you used to differ.
(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:
In verse 92: أَن تَكُونَ أُمَّةٌ هِيَ أَرْبَىٰ مِنْ أُمَّةٍ (merely because a group is higher [ in number and wealth ] than the other), Muslims have been ordered that they should not break the pact they enter into with a group simply for the sake of worldly interests and gains. For example, they may come to think that the group or party with which they have entered into a pact is weak and few in numbers, or lacks financial backing, while the other group or party abounds in numbers and strength, or is wealthy. Now, if they are tempted to join the party having power and wealth in the hope of better gains from them, this breaking of the pledge given to the first party is not permissible. Rather than do something like that, one should stay by the pledge given and let all gains and losses rest with Allah Ta` ala. However, if the group or party with which one has entered into a pact commits or commissions acts contrary to the dictates of Islamic religious law, then, it is necessary to break that pledge - subject to the condition that they be clearly forewarned that Muslims will not be bound by that pact anymore - as stated in the verse: فَانبِذْ إِلَيْهِمْ عَلَىٰ سَوَاءٍ (then, throw [ the treaty ] towards them being right forward - 8:58).
At the conclusion of the verse, the given situation has been identified as a device to test Muslims whereby Allah Ta’ ala tries them to deter-mine if they would obey the desires of their self and break the solemn pledge, or go on to sacrifice their selfish motives in obedience to the com-mand of Allah Ta’ ala.
Tazkirul Quran
Maulana Wahiduddin Khan
The spinning of cotton involves the hard work of combining separate fibres so that articles useful to man may be woven from it. If someone spins cotton, working hard the whole day, but in the evening tears the spun thread to pieces, then the entire hard work will go waste. The same is true of those who enter into a mutual agreement and one or the other party breaks it without any valid reason. Shredding the cotton thread after spinning it amounts to wasting one’s labour. Similarly, the breaking of an executed agreement nullifies the whole process by which a matter of mutual unity had come into existence. In the present world a man spends his life along with other human beings. Every man has to do his work among many other men. For this reason, great importance is given to mutual confidence and trust. In order to maintain this collective social life, agreements and decisions between one man and another man are frequently entered into, sometimes by oath and sometimes without an oath. Now, if people start breaking such agreements without any real justification, social life will be greatly disturbed and constructive development of any kind will no longer be possible. There are two ways of making an agreement in the name of God. In one case a pledge is made formally, uttering the words of the oath. In the second case the words of the oath are not uttered, but reference to God in some form is included in the agreement executed. In all such cases the undertakers of oaths are as if making God the witness or surety in the matter. The breaking of an agreement in which the name of God is also included is much more serious, because this shows that it is only when a man needs to win the confidence of others that he makes use of God’s name, but when he is overcome by the requirements of his own interests, he ignores God. There are two types of agreements between individuals or communities—one is based on principles, and the other is based on interest. In ancient times, the position was that whenever there appeared to be any advantage in making such an agreement, it was executed, but whenever it seemed beneficial to break it, it was broken. Things are no different today. As opposed to this, Islam requires that agreements should be subject to the principles of Islamic law and morality.
Uthmani text of the Madinah mushaf (King Fahd Complex). Translation: Saheeh International. Tafsir via Quran.com.