Is the salary considered unlawful if a person was hired after substituting their urine sample in the medical examination due to prior hashish use, knowing that they have repented from using hashish?

Your action of substituting something other than what you were asked to examine is forgery and prohibited deception, and you must repent to Allah for it by feeling remorse, resolving not to repeat it, and increasing your supplications for forgiveness and good deeds, for the Prophet, peace and blessings be upon him, said: "Whoever deceives us is not of us."

As for the salary, if you are performing the work required of you, there is no sin upon you in it, as it is in exchange for the work you perform.

You have done well by repenting from consuming the prohibited substance that destroys minds, corrupts dispositions, and harms bodies.