Ada Hi 👋,
If it is Sixt car rental, probably the amount you expect is the credit for the security deposit.
If the vendor indeed release the amount for refund also with their payments providers then Master Card will refund the funds and immediately right after bunq will add it to you account.
If the vendor does not do anything, the booking for the security deposit payment will expire with in 30 days from booking date.
If you still have issues, you should contact bunq support via the App. This is a community of users and no one has access to your account.
Regards
GG
