Was the excess amount the questioner received from the company over the value of the airline ticket rightfully his, knowing that he did not give it away in charity?

It is permissible to benefit from the money if the company authorizes you to do so, and if you donate it, you will be rewarded. However, if you know that those in charge of the company are not authorized to dispose of this money, and that what is permitted for you is the actual ticket price, then it is not permissible for you to benefit from it, and it must be returned to the company. It is not permissible to donate it unless it is impossible to return it to its owner.