The mishna continues its list of cases where oaths were instituted by the rabbis. The gemara explains why the worker is believed to say he/she didn’t get paid. However this halacha is qualified as only applying in a case where the time in which the worker should have been paid hasn’t passed yet – once that time passes, there is an assumption that the employer paid the worker. Shmuel and Rav both hold that the worker can swear only if there were witnesses that saw the worker being hired. If not, the employer can claim he/she never hired the worker at all and therefore is believed by saying the worker was already paid because of a “migo.” Rava disagrees with this.