Forced to pay visa cost and terminated for another's mistake. What to do?

Dear Sir/Madam,

I am working in a company on probation for 2 months. Due to a mistake done by the former employee who worked in my position, the employer gave me a termination letter with 30 days' notice without considering my explanation.

Also, the prior employee came to train me for these two months and his payment was deducted from my salary.

The employer asked me to pay more than 6000 AED as visa cost from this month's & next month's salary.

As per the contract letter I signed with him, there is a clause as I have to bare the cost of employment (visa related) if terminated on probation.

Kindly help on this since I have to work without payment till my last working day even though it is not a voluntary resignation but an unfair termination.