Is money received for "overtime" that was not worked, and which the employer justified as being for diligence, considered permissible?

If the employer is the owner of the money, then it is permissible for you to take what he gives you as a gift. However, if the money belongs to someone else, then it is not permissible for you to take it unless there are clauses in the institution that grant additional sums to diligent individuals. Otherwise, it is considered taking someone else's money unjustly, which the Holy Quran has forbidden in His (the Almighty's) saying: "And do not consume your properties among yourselves wrongfully."