WP Query related posts by current page Tag ID

get_the_tags() returns an array of tags name, ID, and more. You should store only IDs in an array, and use it in your query.

$post_tag = get_the_tags ( $post->ID );
// Define an empty array
$ids = array();
// Check if the post has any tags
if ( $post_tag ) {
    foreach ( $post_tag as $tag ) {
        $ids[] = $tag->term_id; 
    }
}
// Now pass the IDs to tag__in
$args = array(
    'post_type' => 'property',
    'tag__in'   => $ids,
);
// Now proceed with the rest of your query
$related_posts = new WP_Query( $args );

Also, use wp_reset_postdata(); instead of wp_reset_query(); when you are using WP_Query();.

UPDATE

As pointed out by @birgire, WordPress offers the wp_list_plunk() function to extract a certain field of each object in a row, which has the same functionality as array_column() function.

So, we can change this:

// Define an empty array
$ids = array();
// Check if the post has any tags
if ( $post_tag ) {
    foreach ( $post_tag as $tag ) {
        $ids[] = $tag->term_id; 
    }
}

To this:

// Check if the post has any tags
if ( $post_tag ) {
    $ids = wp_list_pluck( $post_tag, 'term_id' );
}

UPDATE

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