Withholding salary and passport after visa cancellation

How do I get my salary and passport from my employer?

I worked for my previous company for 3 months and decided to resign because of a better opportunity. I signed an appointment letter that I can resign within the probation period of 6 months.

I resigned on July 15, 2018, and served the 1-month notice period and worked until August 16, 2018.

They cancelled my visa on August 29 but still, they are holding my passport and my remaining salary and the worst is they are asking me to pay AED 13,000 for the visa processing, flight ticket and visa cancellation.

What can I do about this? I need help badly.

Thank you in advance.