Making the Oath Subject to Allah's Will

Question :

What is the meaning of the Hadith of Ibn 'Umar, may Allah be pleased with him, which states that the Messenger of Allah said:
" not broken his oath."

Answer:

Whoever swears upon an oath, saying: "If Allah wills," he has not broken his oath."

The meaning of this Hadith is that if a person swears to something and he adds the words "If Allah wills", and then he breaks the oath, there is no atonement incumbent upon him. Such as by saying: "By Allah, if Allah wills, I will certainly do such-and-such," then he does not do it. Or: "By Allah, if Allah wills it, I will certainly not do such-and-such," then he does it. In this case, there is no atonement incumbent upon him, because he said: "If Allah wills." Based upon this, it is desirable for a person who swears something to link his oath to the words: "If Allah wills", so that if it was difficult for him to fulfill his oath, there would not be any atonement incumbent upon him.

There is another benefit in the one who swears saying: "If Allah wills", which is that it makes the thing he swore easier to accomplish, because he entrusted the matter to Allah, Most High and Allah, the Almighty, the All-Powerful has said:
"And whosoever puts his trust in Allah, then He will suffice him. Verily, Allah will accomplish his purpose. Indeed Allah has set a measure for all things.

Source:
Ibn 'Uthaimin
Fatawa Islamiyah, Vol. 6 Pages 371-372