you really don't want to mix your rendering like that. and there is no need to use :update => 'mypropForm' in the ajax call in your view, as you are doing a page.replace_html.
you're making an ajax request, so instead of redirec_to, you should do
render :update do |page| if @proposal.save page.redirect_to ... else page.replace_html 'mypropForm', :partial => 'myprop_form' end end