Abbas - Tanwîr al-Miqbâs min Tafsîr Ibn ‘Abbâs
(Make not your oaths) your pledges (a deceit) cheating, plotting and trickery (between you, lest a foot should slip) lest you should slip from Allah's obedience as a person's foot slips (after being firmly planted and ye should taste evil) hell (forasmuch as ye debarred (men) from the way of Allah) from Allah's religion and obedience, (and yours should be an awful doom) in the Hereafter.
And do not make your oaths a [means of] deceit between you — He repeats this [here] for emphasis — lest a foot should slip, that is, [lest] your feet [should slip] off the path of Islam, after being steady, having been set upon it, and [lest] you should taste evil, that is, chastisement, forasmuch as you barred [people] from the way of God, in other words, for your refraining to fulfil covenants or barring others from [doing] this, since your custom [once established] will be followed [by others], and there be a tremendous chastisement for you, in the Hereafter.
(And had your Lord willed, all of those on earth would have believed together.) 10:99, meaning, He could have created harmony among them, and there would not be any differences, conflicts or hatred between them.
(And if your Lord had so willed, He would surely, have made mankind one Ummah nation or community, but they will not cease to disagree. Except him on whom your Lord has bestowed His mercy, and for that did He create them.) (11:118-119) Similarly, Allah says here:
وَلـكِن يُضِلُّ مَن يَشَآءُ وَيَهْدِى مَن يَشَآءُ
(but He allows whom He wills to stray and He guides whom He wills. ) Then on the Day of Resurrection, He will ask them all about their deeds, and will reward or punish them even equal to a scalish thread in the long slit of a date stone or the size of a speck on the back of a date stone, or even a thin membrane of the date stone.
The Prohibition on taking an Oath for Purposes of Treachery
Then Allah warns His servant against taking oaths as means of deception, i.e., using them for treacherous purposes, lest a foot should slip after being firmly planted. This is an analogy describing one who was on the right path but then deviated and slipped from the path of guidance because of an unfulfilled oath that involved hindering people from the path of Allah. This is because if a disbeliever were to find that after having agreed to a covenant, then the believer betrayed him, then the believer will have hindered him from entering Islam. Thus Allah says:
(And do not use an oath by Allah for the purchase of little value.) meaning, do not neglect an oath sworn in the Name of Allah for the sake of this world and its attractions, for they are few, and even if the son of Adam were to gain this world and all that is in it, that which is with Allah is better for him, i.e., the reward of Allah is better for the one who puts his hope in Him, believes in Him, seeks Him and fulfills his oaths in the hope of that which Allah has promised. This is why Allah says:
إِن كُنتُمْ تَعْلَمُونَمَا عِندَكُمْ يَنفَدُ
(if you only knew. Whatever you have will be exhausted,) meaning, it will come to an end and will vanish, because it is only there for a certain, limited time.
وَمَا عِندَ اللَّهِ بَاقٍ
(and what is with Allah will remain.) meaning, His reward for you in Paradise will remain, without interruption or end, because it is eternal and will never change nor disappear.
(And to those who are patient, We will certainly grant them their rewards in proportion to the best of what they used to do. ) Here the Lord swears, with the Lam of affirmation, that He will reward the patient for the best of their deeds, i.e., He will forgive them for their bad deeds.
Maududi - Sayyid Abul Ala Maududi - Tafhim al-Qur'an
That is, "Someone,who might have been convinced of Islam, seeing your dishonest conduct should become disgusted and hold back from joining the Believers." For he might argue like this:"As these Muslims are not much different from the disbelievers in their morals and dealings, there is no reason why I should join them."