Does an expiatory offering for an oath become obligatory if a person swore an oath, being certain that he would not do something, then doubted whether he did that thing?

If you swore an oath about something you were certain of, and then it turned out to be otherwise, this falls under the category of a 'vain oath' (laghw al-yameen), for which there is no expiation according to the Maliki school. Similarly, there is no expiation if you doubt whether you have broken an oath, because doubt concerning breaking an oath does not necessitate anything. The fundamental principle is the acquittal of one's responsibility, and certainty is not removed by doubt.