Is expiation obligatory upon one who swore an oath not to masturbate with his hand, then masturbated without using his hand, believing that his oath pertained only to masturbation with the hand?

If the swearer intended by his oath only to abstain from masturbation by hand, then he does not break his oath by masturbating by other means, nor is expiation for an oath incumbent upon him. However, he must repent and abstain from masturbation because it is forbidden.