Hello,
This is expected behavior:
The user has already submitted a withdrawal and it is in pending state, therefore this balance is now locked.
The site’s admin should now go to Tools -> Fiat Withdrawals and mark the withdrawal as done, once the fiat amount is sent to the user.
Alternatively, the admin can cancel the withdrawal and the funds will be again “available”.
The concept of unavailable balance is described in the glossary section of the manual.
with regards