Is an expiatory oath (kaffarat yamin) obligatory upon someone who swore to awaken his brother but found him awake before he could awaken him?

The one who takes an oath does not break it if the subject of the oath becomes unattainable due to a rational impediment that occurred after the oath, without any negligence on his part. An example of this is someone who swore to wake his brother but found him already awake. In this case, the subject of the oath became unattainable due to a rational impediment, which is the obtaining of what is already obtained (tahṣīl al-ḥāṣil). This is supported by the statement of Al-Dardir in Al-Sharḥ al-Kabīr: "And regarding the rational (oath), he breaks it if it was not time-bound and he was negligent, but not if he acted promptly or if it was time-bound."