Is expiation obligatory upon one who doubted his oath to abandon a sin, then became certain of it after committing it?

You must first sincerely repent from the sin and from other transgressions, and from breaking your oath. You must also offer expiation because you are certain the oath was violated. This expiation is feeding ten poor persons, or clothing them, or emancipating a slave. If you cannot find any of these, then fast for three days.