If I have made a mental promise but have not worded it properly so to speak, is it valid or can I ignore this promise? I did this a few years ago and have acted according to it despite not saying it verbally. Now a famous Mufti told me through an email that mental promises or oaths are not binding or valid. I would just like some advice regarding this issue.
You are held responsible for the oaths and promises you have made up in your mind. The verse 225 in Surah al-Baqarah explicitly describes this:
“God does not hold you responsible for your unintentional oaths, but He holds you responsible for what (the oaths) your hearts have earned. God is forgiver and He treats gently.” (al-Baqarah 2:225)
According to the verse, speaking out the oath is not essential. It is enough that you have made a firm intention in your heart. Although you do not make it verbally, God knows what is inside your heart. So, we advise that you keep on acting according to your promise.
If you have to break your promise because there is a better way now or there is another vital reason, then you should expiate for it. Please see the following about the expiation for breaking an oath or vow: