An employer is not allowing an employee to leave without paying visa fees

Dear sir/madam,

I have been working in Dubai for over 6 months in a security company but now I want to change my company or leave this company.

My company didn't allow me to do it. They asked me to first pay the visa fee and leave. Should I pay for it?

I have already paid when I came here. I have already paid in my country for the visa, and flight ticket. Now, what to do?

Please give me helpful advice!