وَمِنْهُم مَّنْ عَٰهَدَ ٱللَّهَ لَئِنْ ءَاتَىٰنَا مِن فَضْلِهِۦ لَنَصَّدَّقَنَّ
Wa minhum man ʿāhada'llāha la-in ātānā min faḍlihī la-naṣṣaddaqanna wa la-nakūnanna mina'l-ṣāliḥīn · Fa-lammā ātāhum min faḍlihī bakhilū bihī wa tawallaw wa hum muʿriḍūn · Fa-aʿqabahum nifāqan fī qulūbihim ilā yawmi yalqawnahū bimā akhlafu'llāha mā waʿadūhu wa bimā kānū yakdhibūn · A-lam yaʿlamū anna'llāha yaʿlamu sirrahum wa najwāhum wa anna'llāha ʿallāmu'l-ghuyūb
"And among them is one who made a covenant with God: 'If He gives us of His bounty, we shall certainly give in alms and shall certainly be among the righteous.' · Then when He gave them of His bounty they were miserly with it and turned away, averse. · So He fixed hypocrisy in their hearts until the day they meet Him, because they broke with God what they had promised Him, and because they were lying. · Did they not know that God knows their secret and their whispering, and that God is the knower of all that is hidden?"
The promise is reinforced with two lāms and two nūns: la-naṣṣaddaqanna wa la-nakūnanna. In Arabic this (the lām of emphasis plus the nūn of emphasis) is the strongest reinforcement an oath can have.
I record this as an observation of grammar: the word is given in the most definite form language can give it. And it is not kept. The verse, that is, shows by the sentence itself that reinforcement is no guarantee.
I record this as my own reading, and its ground is the verb itself: hypocrisy is set down here not as a beginning but as a consequence. The order is this: the word was given → it was not kept → hypocrisy took root.
This coincides with the order treated in al-Munāfiqūn at 63/3: bi-annahum āmanū thumma kafarū fa-ṭubiʿa ʿalā qulūbihim — there too the sealing of the heart came after an act. I rely on that treatment.
The sense of the root: the back, being behind; and ikhlāf — not standing behind one's word, not keeping a promise given.
The root was treated in detail in al-Fatḥ by way of mukhallafūn, and it will recur with various derivatives in verses 81, 83, 87, 118 and 120 of this sūra. The distribution of the root within the sūra will be given all together at verse 118.