Calculation of the End of Service Gratuity upon termination

I am looking for advice on how to best proceed with receiving the amount of gratuity promised in my contract. I have worked for my employer for 4 years on an unlimited contract.

One month before my visa renewal, the employer informed me that they will not renew my visa and will end the working relationship. My contract states that my basic salary is AED 13,000, with no subdivisions.

Now, the employer claims that this was a mistake and they meant to write AED 6,000 as basic salary and allocate the rest for travel, living etc. They are only willing to pay the gratuity based on this amount.

Please advise, what is the amount the employer owes me as gratuity and how to proceed?