Link post thumbnail using metadata

I’m working on a site and the client wants to be able to add logos to the footer that link to external websites. I set it up so only post thumbnails display in the footer. I’m having trouble getting the thumbnail to link to a custom URL that’s saved in wordpress custom fields metadata called “footer_url”. Any idea what Im doing wrong or possibly have a better way of setting this up?

<?php
			$footer_url = get_post_meta('footer_url');
            $thumbnails = get_posts('category_name=footer-logos&showposts=4');
            foreach ($thumbnails as $thumbnail) {
            if ( has_post_thumbnail($thumbnail->ID)) {
            echo '<div class="footer-logo">';
            echo '<a href="' . $footer_url . '">';
            echo get_the_post_thumbnail($thumbnail->ID, 'thumbnail');
            echo '</a>';
            echo '</div>';
            }
            }
        ?>