Have you tried anything with
The following will have the same net effect as
truncate, but uses a different calling sequence and only works on ext4 and XFS with files that are a multiple of the filesystem block size:
fallocate -c 0 -l `wc -c <file>` <file>
And this will make the file a sparse file of the same (apparent) length as the original file (but only works on ext4, XFS, BTRFS, and tmpfs):
fallocate -p 0 -l `wc -c <file>` <file>
Aside from that, the
unlink command might work (but I doubt it as you commented
find -delete doesn't work.
Based on what you're saying though, it sounds to me like they're using BTRFS for backend storage, which they should probably not be doing if they need to use quotas (quotas on BTRFS work differently than they do on most other filesystems, and have some pretty significant scalability and usability issues right now).