Is it permissible for an accountant to claim transportation costs while owning a taxi, without the employer's knowledge?

If the amount designated by the employer for transportation is fixed and does not depend on the mode of transport used, there is no harm in benefiting from it, even if you use your private car. However, if the amount is determined based on the actual costs of transportation, it is not permissible to claim more than those costs, and any excess must be returned to the company.