Is it obligatory to fast in expiation for an oath (kaffarat yamin) for someone who vowed to abandon a sin, then intended to commit it, then an external impediment prevented them, and they repented?

"What one thinks to oneself" (حديث النفس) incurs no sin and has no consequences, as long as it is not uttered. This is based on the saying of the Prophet, peace be upon him: "Indeed, Allah has forgiven for my nation whatever they think to themselves, as long as they do not speak of it or act upon it." If you did not utter the vow, it is not considered an oath or a vow. Even if you uttered it, nothing is binding upon you as long as you have not committed the sin. However, a firm resolve to commit a sin is recorded as a bad deed with Allah if the person exerted effort to achieve it, but was prevented from it by a reason beyond their control.