you can't free up that capacity, the iSCSI LUN has "balooned" to take up the free space in the file system. Just because you deleted data in the iSCSI LUN, it will not automatically shrink the iSCSI LUN (thus freeing up space in the file system). There is no such thing as VAAI thin block reclaim/UNMAP on Celerra that would release any unused blocks back to the file system.  When I used to deploy iSCSI LUNs on Celerra, i always had a dedicated file system for each LUN so i am curious why are you concerned about freeing up that space ? Even if you were able to get that space released, you still would not be able to release it back to the storage pool as you can't shrink file systems.
Is there either a scheduled task or a manual method for "releasing" capacity within a Celerra file system? In this case, an iSCSI attached LUN filled up which in turn caused the file system to fill up. A bunch of stuff was deleted in the LUN, but the file system is still full. I haven't been able to find any documentation that speaks to this specifically. Let me know if you need any other details. Thanks
Release Celerra File System Capacity