Shaikha Umm Abdillah al-Waadi’yyah (may Allah preserve her) said:
My father Shaikh Muqbil (may Allah have mercy upon him) said, “(إن شاء الله) is a blessed statement. It is not said in regards to something except it is achieved in most cases” or with this meaning.
From the benefits of (إن شاء الله) is that if someone makes an oath and says: (إن شاء الله), then he would not have broken his oath if he did not do it.
From its benefits is that if he says, “I will do such and such (إن شاء الله) tomorrow”, and if he does not do it, there is nothing upon him. So like that, Musa said to Khadr (may peace be upon both):
“You will find me, if Allah wills, patient, and I will not disobey you in [any] order” [18:79].
He was not patient (in regards to that), but he is not considered a liar nor one who broke his promise because he said: (إن شاء الله)
From its benefits is remembering Allah, the Exalted and seeking blessings by remembering Allah”.
Faisal bin Abdul Qaadir bin Hassan
If someone says (إن شاء الله) and does not intend to do the action or fulfill his promise, then it is considered lying and breaking a promise. However, if someone says it and intends to do it, but an excuse prevents him from doing so, then he is excused إن شاء الله.
Hafidh Ibn Rajab (may Allah have mercy upon him) said: “The second [characteristic of a hypocrite is]: ‘if he promises, he breaks it’. It is of two types: one of them: he promises and his intention is not to fulfill his promise, then this is the worst of breaking promises. If he said: ‘I will do such and such (إن شاء الله تعالى)’ and from his intention is not to do it, then this is a lie and a broken promise. al-‘Awzaai (may Allah have mercy upon him) said this.
Second: he promises and from his intention is to fulfill it. Then he is set to do it, but he breaks the promise without a reason. This is breaking a promise [as well]…” [Jami’ Uloom wal-Hikam (pg. 559)]
So this is not from the category mentioned earlier because Prophet Musa (may peace and blessings be upon him) was resolved to be patient while with Khidr. As Imam as-Sa’adi (may Allah have mercy upon him) said in the Tafsir [18:79]: “This is resolve from him (Musa) before the presence of something which tested it (his resolve)…” [Tafsir as-Sa’adi]
And Allah Knows Best
Faisal Ibn Abdul Qaadir Ibn Hassan