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.
If the case is that your company (employer) gives you money for transportation and gives you this money to dispose of as you wish and you find someone who gives you a ride for free and then you take the money for yourself and the company does not oblige you to give it back and does not ask you how much you paid for the transportation and whether or not you paid anything, then it is permissible for you to save some of this money and benefit from it for your own personal and permissible use. "What is known by custom is like a condition in what is made conditional" as stated in the rules of Fiqh. This means that what people are used to in their dealings is considered like a condition in regard to adhering to it and complying with it, even if this is not explicitly mentioned, as was stated by the author of Al-Wajeez fi Eedhaah Al-Qawaa’id Al-Fiqhiyyah.
However, if your employer requires you to pay the transportation fares and return whatever remains, then you have no right to hide this from him.
If you are in doubt, then you may ask the responsible person in your company about this.
Allaah knows best.