I\'d like to work with pages without trailing slashes. So now I want my URL\'s with an trailing slash to redirect (using .htaccess) to the same URL without the trailing slas
Wordpress redirects depending on what your permalink structure is. So any changes to .htaccess helps little since Wordpress will add/remove them for you and peform a wp_redirect() after .htaccess done things. In worst case you end up with a redirect loop.
A solution is to turn off Wordpress redirecting using.
// perhaps an if(is_something()) before here ...
add_filter('redirect_canonical', '__return_false');
You should wrap this inside an if statement or such where you perform a check for a certain page/directory/etc. Putting the above directly in your functions file will turn off wordpress redirections and probably break things.