Abbas - Tanwîr al-Miqbâs min Tafsîr Ibn ‘Abbâs
(And purchase not a small gain at the price of Allah's covenant) i.e. swearing by Allah falsely to obtain a small worldly gain. (Lo! that which Allah hath) of reward (is better for you) than what you have of wealth, (if ye did but know) the reward of Allah; it is also said that this means: if you believe in the reward of Allah.
And do not sell God’s covenant for a small price, of this world, so that you break it in return for this [small price]. For truly what is with God, of reward, is better for you, than all that is in this world — if you should know, this, then do not break it.
(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, "The Covenant that you may make in the name of Allah as a representative of His religion."
It does not mean that they should barter away Allah's Covenant for some big gain. What it implies is that any worldly gain howsoever great, is insignificant as compared with the worth of Allah's Covenant. Therefore, it will be a losing bargain to barter that away for any worldly gain, which is after all paltry.
Those who practice fortitude are the people who will always stand Finn in the struggle between right and truth on one side and greed and lust on the other. They bear every loss for the sake of righteousness and spurn away every gain that they might obtain by adopting unlawful means. They patiently wait for the rewards of their good deeds in the Hereafter.
This verse removes the wrong notions of both the Muslims and the disbelievers who are of the opinion that those who adopt a just honest and pious attitude, are most surely losers in this world, though they might be gainers in the Hereafter. Allah removes this misunderstanding, as if to say, "This presumption of yours is wrong. The righteous attitude not only leads to a happy life in the Hereafter, but it also guarantees, by Allah's grace, a pure and happy life even in this world. " And this is a fact: those people who are sincerely righteous, honest, pure and fair in their dealings. Enjoy a much better life in this world, for they enjoy that confidence and real honor and respect because of their spotless character, which is not enjoyed by those who lack these virtues. They obtain such pure and outstanding successes as are denied to those who employ dirty and disgusting ways to win success. Above all, they enjoy, even though they might be living in huts, that peace of mind and satisfaction of conscience which is denied to the wicked dwellers of mansions and palaces.
That is, "Their rank in the Hereafter shall be determined according to their best deeds". In other words, "If a person has done both small and great virtues," he will be awarded that high rank which he would merit according to his greatest virtues.