If you use the Claim Fax button (on the Browse Received Fax window) and claim a fax that belongs to another user you cannot simply "unclaim" it. You must transfer the TIFF file to the user. You can do this by saving the fax as a file and either copying the file to the other user's directory.
If you have a mail system that supports binary attachments, you can email the TIFF file to the other user.
For instructions on how to save a fax to a TIFF file, see Saving a Fax to a File.