Have you had a good look around the base of the posts? I'm wondering if he really has put dry mix (ballast and cement) in there. What's in there at the minute might have a bearing on how you try to solve this. As Andy says some pictures would help including pictures of the base of the posts, with any topsoil at the base of the posts scraped back.
This page gives advice on fence installation: http://www.pavingexpert.com/featur02.htm. It would be worth reading that before commissioning or carrying out any more work. As people have already said, lots of people use fast setting postcrete type product, but that doesn't mean dry mix is wrong or won't work.
http://www.pavingexpert.com/featur02.htm
This page gives advice on fence installation: http://www.pavingexpert.com/featur02.htm. It would be worth reading that before commissioning or carrying out any more work. As people have already said, lots of people use fast setting postcrete type product, but that doesn't mean dry mix is wrong or won't work.
http://www.pavingexpert.com/featur02.htm