give read&write permission to only 1 file in folder
To allow user_name writing to preferences.plist add an ACL with:

sudo chmod +a "user_name allow write" /Library/Preferences/SystemConfiguration/preferences.plist


This changes the output of:

ls -laeO /Library/Preferences/SystemConfiguration/preferences.plist
-rw-r--r--  1 root  wheel  - 59715  9 Feb 23:25 /Library/Preferences/SystemConfiguration/preferences.plist


to:

ls -laeO /Library/Preferences/SystemConfiguration/preferences.plist
-rw-r--r--+ 1 root  wheel  - 59715  9 Feb 23:25 /Library/Preferences/SystemConfiguration/preferences.plist
0: user:user_name allow write


but doesn't change the permissions of the superior folder or other files residing in the same folder:

drwxr-xr-x  15 root      admin        -             510 24 Mär 17:23 SystemConfiguration


To remove the ACL later simply enter:

sudo chmod -a "user_name allow write" /Library/Preferences/SystemConfiguration/preferences.plist