Is it permissible to report someone who is absent from his work, receives his full salary, and at the same time works in another place during official hours?

First, the offending person must be advised and reminded of the necessity of adhering to the concluded contract, and that they will be reported if they do not comply. If they do not take heed, it is permissible to report them as a form of denouncing evil (munkar), and this is not considered backbiting (gheebah) which is forbidden.