What must the questioner do to absolve himself of the shortcomings he committed in his previous work?

What you mentioned about your negligence and deception towards your workplace necessitates seeking forgiveness (istighfar), repentance (tawbah), and seeking absolution (tahallul). Allah Almighty said: "O you who have believed, fulfill [all] contracts," and the Prophet, peace and blessings be upon him, said: "Whoever cheats us is not of us."

Absolution is achieved by returning what you took from the salary without a rightful claim to the company, or by requesting exoneration from someone who has the authority to waive the right. Merely leaving the job is not enough; rather, absolution from the company's right is essential.