Is it obligatory to inform the company about using the discounted transportation card, and is the difference in amount considered unlawful or rightful gain?

The reference for determining eligibility for full fare despite having a discount card is the company's terms and conditions, based on the Prophet Muhammad's (peace be upon him) saying: "Muslims are bound by their conditions, except a condition that makes a lawful thing unlawful or an unlawful thing lawful." If the company pays the actual transportation fare, it is not permissible to take the difference. However, if it pays a fixed amount regardless of the actual cost, there is no harm in taking the difference. To clear one's conscience and avoid consuming unlawful earnings, the truth must be disclosed to the company.