Can't add to $args $_GET value conditionally - WordPress - wordpress

Question

In WordPress I have a loop with some $args array, which I want to be able to edit conditionally depending on a $_GET value.
$args = array(
'post_type' => 'property',
'post_status' => 'publish',
'posts_per_page' => -1,
'meta_query' => $meta_query,
'tax_query' => $tax_query,
'fields' => 'ids',
'no_found_rows' => true,
'post__in' => $post__in,
'nopaging' => true
);
When adding to $args array like this:
if ( $_GET['prop_sort'] == 'price' ) {
$args['orderby'] = 'meta_value_num';
$args['order'] = 'ASC';
$args['meta_key'] = $prefix . 'price';
}
Nothing changes, and added to $args stuff gets ignored. However when I put code like this:
args = array(
'post_type' => 'property',
'post_status' => 'publish',
'posts_per_page' => -1,
'meta_query' => $meta_query,
'tax_query' => $tax_query,
'fields' => 'ids',
'no_found_rows' => true,
'post__in' => $post__in,
'nopaging' => true
);
$args['orderby'] = 'meta_value_num';
$args['order'] = 'ASC';
$args['meta_key'] = $prefix . 'price';
Everything works as intended and posts are sorted by price from small to big.
What could be the problem?
EDIT:
Trying to influence query like this:
function myplugin_pre_get_posts( $query ) {
if ( ! is_admin() && $query->is_main_query() && $query->query_vars['post_type'] == 'property' ) {
$query->set( 'orderby', 'meta_value_num' );
$query->set( 'order', 'ASC' );
$query->set( 'meta_key', 'sb_price' );
}
return $query;
}
add_action( 'pre_get_posts', 'myplugin_pre_get_posts', 1 );
Dones't work either (Undefined index: post_type in ..\functions.php on line 65).