Is expiation (kaffarah) obligatory for someone who swore to abandon a sin and said, "If I return to it, I will shave my mustache," while being able to shave it?

If a person swears by Allah that he will not return to a sin, and if he does, he will shave his mustache, then he returns to it: If he set a time for shaving his mustache and did not do it, then expiation for breaking an oath (kaffarat yamin) becomes binding upon him. But if he did not set a time, he does not break his oath unless he becomes unable to shave his mustache. He may also break his oath himself and offer expiation for breaking an oath.