Commentary on Tafsir al-Jalalayn: ((The promise of Allah)) is in the accusative case due to its implied verb ((Allah does not break His promise)) and His promise.

The Shaykh : Allah the Exalted said: The author states that "promise" is in the accusative case due to an implied verb, meaning "they were promised the promise of Allah." Or, according to another possible view, the implied verb is "they fulfilled the promise of Allah," meaning Allah fulfilled His promise for them. On this view, it is in the accusative case due to an implied verb other than the explicit one. As for the author's view, it is a deleted noun with its implied verb. May Allah have mercy on him. He said: Allah the Exalted said: The commentator author states "His promise," which indicates that the "al" here takes the place of the pronoun, and that "al-mi'ad" (the promise) means "al-wa'd" (the promise). And the statement means that He does not replace what He promised with anything else, because "akhlafa" (to replace) indicates replacing one thing with another, as Allah the Exalted said: Unlike "khalafa" (to succeed), which indicates following one thing with another. So one says "he succeeded him," meaning he came after him. "Akhlafahu" means to make a substitute for him. For this reason, the one who has suffered a loss says, "O Allah, reward me in my loss. No, but replace it for me, and replace it for me with something better than it," meaning give me a substitute for it. And the statement "Allah does not break His promise" is so because of the perfection of His truthfulness, and as you have implied, because breaking a promise is either due to the liar's lie or due to his inability. And Allah Mighty and Majestic is free from this and that; He is perfect in truthfulness and perfect in power.