The cashier refuses to sell a free ticket to a war invalid

Yes, legitimate.

There is such a concept as the monetization of travel, that is, payment of the benefit of free travel with money. You can receive compensation through the social security body at the place of registration of the person. If you receive a pension or social benefits through a social security agency, the funds from the monetization of the fare will be automatically transferred to your account.

If you live in an area where monetization is not implemented, in this case it will be necessary to purchase a ticket, to indicate on it the name of the driver or other person who accepted the payment; affix the signature of the driver or other person who accepted the payment; the stamp of the company that carries out the transportation, as well as indicate the date of the trip, the name of the settlement and apply to the local self-government body at the place of residence with an application for reimbursement of travel expenses, attach a ticket to the application.

If you did not like the cashier's service, you can file a complaint with his management.