In my company executives has a privilege to hire taxis when the company vehicle not available for office use. After spending for the taxis they simply put a voucher and reimburse the money they spend. Certain executive use this facility in a different way. They use public transport and claim the taxi fares (e.g. certain distance that taxi fare is 1200/- whereas the public transport cost only 100/-) which they claim is not wrong as it is their privilege to enjoy and they can use it the way they wants it. What is your opinion?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.

If the regulations of the company where these employees work entitle them to transport fees as a part of the wages that is agreed upon with the company, meaning that the company offers them a transport bonus, then they can use any kind of transportation and keep the difference. But if the company authorizes them to buy a ticket, and this is what appears from the question, then they are obliged to give back any excess money, as in this case they are agents and any excess money is to be given to the authorizer not to the agent. 

Allaah knows best.