I don't think that you should use the Link field in your case, if you want to create relation with post object...
It would be much better to use Relation field, if you should be able to select many posts, or just Post Object, if it should be possible to select only one value.
If you go for the Post Object field type, then remember to set its Return Format to Post ID.
get_sub_field( 'your-field-name' ) will return exactly the ID of selected post.
If you want to display link to this post, you can always use
echo get_permalink( get_sub_field( 'your-field-name' ) );
So this is your code after modifications (I've used your original name for the field, but it would be nice to change it too, I guess, since it doesn't contain link anymore):
$post_id = get_sub_field('offer_link');
if ( $post_id ):
<a href="#post-<?php echo esc_attr($post_id); ?>"><?php echo get_post_field( 'post_title', $post_id ); ?></a>
<?php endif; ?>