Samba – Permission issues with Samba and OS X

I'm having some serious permission issues with Mac client machines (OS X Snow Leopard) connected to a UNIX server (running CentOS) through Samba. No matter which permissions I set for a file on the server, the Mac ignores them and changes the permissions back to 766.

My goal is to be able to lock a file (744) from other users when its in use or open on a client machine. Other clients should be able to read the file during this time, but not be able to write to it. I found a thread that describes my problem almost exactly suggesting to use the "unix extensions = off" parameter in the global section of the smb.conf file, but to no avail. The parameter is rejected by the "testparm" command on the the version of Samba I am running (3.4); The compiling issue is resolved by changing the parameter to "unix extensions = no" but still fails to resolve the issue.

The same issue arises when I attempt to use AFP.

I'm really unsure of where to turn next, does anyone have any suggestions or information I can utilize to solve this problem?

Thanks!