Is the salary received for a job you have not started and a period you have not worked lawful or unlawful money? And what is the ruling on spending this money on medical treatment for an illness?

An employee is a private hire (ajīr khāṣṣ) who is entitled to their wage by making themselves available to the employing entity, even if there is no work. So, as long as you are ready to work and have made yourself available to the entity, you are entitled to the salary for the previous period, and you may use it for any permissible purpose.