Sixt The Hague Refuses to Return €400 Deposit After 50 Days

I rented a car from the Sixt branch in The Hague, Netherlands, from December 2 to December 7, 2025. At the time of rental, a €400 deposit was charged to my debit card. The staff member there explicitly told me the deposit would be returned immediately upon returning the vehicle without any issues. I returned the car on time and in perfect condition — yet nearly 50 days later, the deposit still has not been credited back to my account.
During this time I reached out to them by email repeatedly, and every single time I received nothing more than "we are working on this" — no concrete action, no refund. I refuse to accept my €400 being held for this long when there was no damage, no additional charges, and no legitimate reason to withhold it.
I find it completely unacceptable that the Sixt branch in The Hague has treated me this way and continues to hold my money. I am demanding that my €400 deposit be returned to me in full without further delay, along with a clear, written explanation of why it has been withheld for so long.