HP-UX

Hey Brother...

change the file system permissions and expoert only file system from serverA to Server B

No you cant...
Only way I see ( and do sometimes...) is to create a directory called serverB in the filesystem present on that logical volume and use that as source to be mounted then on serverB it will show up as empty mount point...
The issue I see is you cannot avoid serverA to see the content and it is wise someway for the safest way to save the data stored here would be by serverA backups...
You can try to limit access by changing permissions on this directory and change owner and group to someone known both sides

As I understand u need some data space on server-B from server-A via NFS ...
####
Let Server-A have partition
/export -- free about 100 GB (In your case let /dev/vg02/exportdump is mounted on /export/)

Create a dir in /export/A and export it over NFS ..yOU CAN'T directly export /dev/vg02/exportdump

Server-B have partition
/export_dump which ios 30GB AND is almost full (You require to use free space of Server-A to be used over here)

- Now say u have a sub-dir /export_dump/backup whic is utilizing most space say 10GB
## Rename this dir
# mv /export_dump/backup /export_dump/backup_1

## Craete dir
# mkdir /export_dump/backup (ensure to retain if there any specific user/group permission on original /export_dump/backup had)

## Mount
# mount -F nfs serverA:/export/A /export_dump/backup
## make above entry in fstab for permanaent

## Now copy or mv all files from /export_dump/backup_1 to /export_dump/backup
# cp -rp /export_dump/backup_1/* /export_dump/backup/

## Delete /export_dump/backup
# rm -fr /export_dump/backup_1

## Now check
# df -PK /export_dump
# df -Pk /export_dump/backup

See the difference ...