Each office needs to have a SAMBA share dedicated to their office. All members of an office must have read and write permissions on the share for theirf office, but they also must not be able to clobber the permissions or ownership (you will need to setuid group and possibly even user).
Each share should be owned specifically by the head of the associated office.
Make the SAMBA shares located in
/home/shares/pr with a share name of 'pr' and
/home/shares/sales with a share name of 'sales'
Once you have done this, connect to these shares with the various users and verify that they have the correct permissions.