What is the expiation for someone who swore to do something they did not intend, then remembered that they did not do it?

You are not required to offer expiation for swearing an oath concerning a past event, according to the majority of scholars. Nor is there any sin upon you, as long as you did not intentionally lie and were relying on your predominant assumption. Therefore, an oath concerning a past event does not necessitate expiation, whether you were lying or truthful. Its maker is only sinful if he intentionally lied or if that was his predominant assumption.