Passing POST data from one WP post to another

You need to register the query var so it doesn’t get stripped by WP. Add this to your functions.php file.

function foo_add_query_var($vars) {
    $vars[] = 'discount';
    return $vars;
}
add_filter('query_vars', 'foo_add_query_var');

To call this in your template, simply use the following:

$discount = get_query_var('discount');