What can I do if my ex-employer claims money from me?

I would need legal advice in relation to the 30% ruling.When I moved to the Netherlands and started working in December 2019, my then-employer and I applied for the 30% ruling. I filled out the form and they submitted the document package. We received a positive decision from the tax authority and I received the benefit 22 months, until I left the employer and started a new position at my current employer in October 2021. With my current employer we applied again and this time received a negative decision from the tax authority, saying that my annual salary does not reach the required income threshold. I knew my annual salary actually reaches the required threshold, so I went back to my previous employer for clarification. Turns out they miscalculated the amount of benefit, instead of calculating on the income above the threshold, they calculated it on my whole annual salary. Apparently, this resulted in a higher benefit than it was supposed to be. The about €6500, that they want me to pay. To correct the miscalculation my then-employer submitted the correction to the tax authority and paid the above mentioned amount. However, they expect me to pay them this amount. I would need advice on my options here. I find it unfair to expect me to pay this amount. The application of the 30% ruling was part of my salary negation back in 2019, and I accepted their offer counting on the benefit. Now, I feel it all has been negated if I had to pay back €6500. I would really appreciate your help and support. Kind regards,