Answered by Ustadh Tabraze Azam
Question: I was in love with a girl and we used to talk on phone. In the final semester of my school, I fell short of my attendance and my professor didn’t agree to clear my attendance. So I said to Allah, “O Allah, please bring me out of this problem and I promise that I will stop talking to that girl on phone until our marriage”. Allah responded to my prayer and my problem was solved.And we stopped talking for few days. But soon after that we started talking again.Time has passed on and I have sincerely repented to Allah for breaking my promise. My question is that do I have to pay any expiation to compensate for the breaking of that promise or is the repentance is enough?
Answer: Assalamu alaikum wa rahmatullahi wa barakatuh,
I pray this finds you in the best of health and faith, insha’Allah.
The wording used does not appear to be an oath (yamin).
Though, in general, the ruling of “promising to Allah” would be considered according to the custom (`urf) of a particular place. Thus, if it is used for oaths (yamin), it would be considered as such. If not, it would be considered a promise (wa`d).
In any case, it is important to note that what is considered is a verbal utterance of the phrase. An internal  promise (wa`d),  vow (nadhr) or,  oath (yamin) is of no legal implication.
The Expiation for Breaking an Oath (yamin)
The expiation (kaffara) for breaking an oath (yamin) is to [a] feed ten poor people (2 meals each) or, [b] to clothe them (1 garment each) or, [c] to give them the equivalent monetary value. If one is unable to do that, one must fast three consecutive days.
[Nahlawi, al-Durar al-Mubaha; Haskafi/Ibn `Abidin, al-Durr al-Mukhtar/Radd al-Muhtar]
Please also see:
Violating a Promise to Allah and:
What is the Difference Between a Promise, an Oath, and a Vow? and:
Is There a Difference Between Breaking an Oath and Breaking a Promise?
And Allah knows best and He alone gives success.
Checked & Approved by Faraz A. Khan