Problem with file permission

[Paolo Borsa]

Hi all,
I've this problem:

I've a file on my linux system (/tmp/bogus.txt) with permission 775 and ownership borsa:users. /tmp directory is shared with samba 2.0.6

User luigi:webuser, with additional group users, open bogus.txt with windows (with notepad.exe) and modify it. At this time, bogus.txt become owned by luigi:webuser.

Now I (borsa:users) want to modify bogus.txt, but I've no choice to do it, because I've no additional group. If I log in windows with administrator (root) privileges, and modify this file, it become owned by root:root....

There is a way to leave file ownership unchanged? And why this happen? If I modify a file in unix (if I have the rights to do it) that file never become mine...

Any idea?

Thanks in advance, and sorry for my poor english...

Paolo Borsa