Is it possible to quit a job before completing six months?

Dear Sir/Madam,

I work as a stockbroker at a company, but I want to quit because it is not a good job. I have been working there for over four months.

If I ask to leave, the company demands 5000 AED and insists that I complete six months before they agree to my resignation, otherwise, they will ask for the money.

What should I do?