What is the ruling on an employee designating a distant city as the destination for his travel during his leave, without actually going there, in order to benefit from the difference in the ticket price?

This act is a form of cheating and lying, and it is not permissible to engage in it. Rather, a person must write down the true destination they intend, otherwise, they would be devouring people's wealth unlawfully, as Allah Almighty says: "O you who have believed, do not consume one another's wealth unjustly."