Facebook's sharer.php is no longer deprecated but I suggest you switch to using dialog/feed to steer folks back to your coupon.
<a id='fb-share' style='text-decoration:none;' type='icon_link' onClick="window.open('https://www.facebook.com/dialog/feed?display=popup&app_id=YOUR_APP_ID&link=THE_PAGE_WITH_THE_SHARE_BUTTON?&description=MY_AWESOMENESS&redirect_uri=THE_PAGE_WITH_THE_COUPON&picture=URL_OF_A_PRETTY_IMAGE', 'mywindow', 'toolbar=no,status=no,menubar=no,scrollbars=yes,resizable=yes,width=580,height=325');
return false;" href="javascript: void(0)">Share on Facebook</a>";
I can not find a TOS violation for offering coupons to those that like or share.