Is a promise made by a man just saying ‘ i will bring u this or that’ or is a promise made when he/she adds ‘ i promise to bring you this or that or wallahi i will bring you this or that’? Basically is a promise made without using the word promise or wallahi, as one has to give expiation for breaking that.
If one promises another person to do something, it is an issue of dispute whether he is sinful for not fulfilling his promise or not.
The most authentic opinion is that he is not sinful if he intended to fulfill and couldn’t! But if he intended from the beginning to break it, he is sinful.
As long as he didn’t take the name of Allah, there is no expiation in that.