This isn't really my area of expertise but, since I see no one else has responded yet, I do have a couple of thoughts. Be kind if they don't apply well. :)
First, with the file can you change the "owner" to the user that will be accessing via Samba? Even just to test this may help to verify if "full control" isn't enough in this case. The post at
http://www.techsupportforum.com/microsoft-support/microsoft-office-support/253853-powerpoint-2007-file-access-samba.html is partially why I suggest this.
Another thought is will the computer sharing the file run OpenOffice? It would be nice to know if the computer can open the file. Not the best test since it isn't Powerpoint but could still help. I believe this machine is probably Linux but I am pretty sure the OpenOffice programs are supported on that OS and will handle the Powerpoint file.
The final though, and more because of my expertise, is two-fold. Does Samba need to be configured for file types. I know web servers (my area) often do and when I see an error like this in those cases that is often the cause.
Another (possibly "off the wall") thought is what are the permissions for a new file on that folder created by the user running Powerpoint. Powerpoint will try to create a "temp" file when you open a file. I couldn't find a way to prevent this in Powerpoint but I had the thought the problem may be in that step (i.e. can't create the file or permissions on the new one wrong).
I hope this helps. If you have a question I will be glad to try to help more. Let me know how this works or your thoughts.
bol
