Change:
the_post_thumbnail( 'offer_slider' );

to

echo "<a href='" . get_post_meta($post->ID, 'doover_link', true) . "'>";
the_post_thumbnail( 'offer_slider' );
echo "</a>";


Change divs to:
 <div class="title" style="left: 550px;">
 <div style="width:400px; top:0px; left: 0px; position:absolute; z-index:1;">
...

<div class="desc" style="margin-left: 550px;">
                                        <div style="width:400px; top:170px; margin-left: 0px; position:absolute; z-index:4;">