Must the head be completely shaved in fulfillment of a vow that one playfully obligated oneself to?

A vow to shave one's head is considered a vow concerning a permissible act, which is not binding according to the Shafi'is, Malikis, Hanafis, and Zahiris. It is binding according to Ahmad, but they do not obligate its fulfillment. It is preferable to abandon it and offer expiation, because shaving the head for other than ritual purposes is disliked, and expiation for an oath is a way out of the vow.