fetch custom post by meta key if key value is does not know

by

You have to use NOT EXISTS. You will get all posts, which do not have meta field ‘seasonaldiscountdesc’.

$args = array(
    'post_type'         => 'mad_property',
    'post_status'       => 'publish',
    'paged'             => '-1',
    'meta_query'  => array(
        'key' => 'seasonaldiscountdesc',
        'compare' => 'NOT EXISTS'
    )
);