What is incumbent upon one who swore to his manager that his colleagues were receiving an allowance while he did not know whether they were receiving it or not, and he himself received this allowance?

Embarking on an oath while being uncertain about the subject of the oath constitutes a "yamin ghamus" (a false oath), which is one of the major sins that plunges its perpetrator into Hellfire. Expiation (kaffarah) is not effective for it; rather, repentance is obligatory. Whoever has committed this must repent to Allah and dispose of the money they acquired based on this oath, either by seeking forgiveness or by returning it to the employer.