Hi Samira,

i'm not sure to fully understand what you wronte and what you are trying to do.
However the return URL page is not determined by the href attribute, so this makes no sense:

href="'.$return_url.'"

The return URL is managed by the component settings and module PHP code as for the default Joomla login module. Choosing 'default' in the backend settings of the component as 'Login URL' will cause to redirect back users to the same page used for the login.

Cheers,
John