Clicking delete button a check is run whether the set-off has a status
assigned to it: posted,
confirmed, rejected or reversed, and accordingly, the following message is
returned:
Click 
to close the window.
Solution:
1. Unable to delete set-off proposal because it was already posted,
confirmed, rejected or reversed.
2. Trying to delete a journal entry with which the set-off was posted, its
status changes to in preparation . In such a case you are able to delete
set-off.