Solution:
add_action( 'woocommerce_order_status_completed', 'nach_dem_Kauf', 10 , 1);
function nach_dem_Kauf($order_id) {
$order = wc_get_order( $order_id );
$items = $order->get_items();
foreach ( $items as $item ) {
$total = $item['total'];
$product = wc_get_product($item['product_id']);
$arr=get_post_meta($product->id, 'wpneo_funding_maximum_price');
$arr[0] = $arr[0] - $total;
'wpneo_funding_maximum_price', intval($arr[0]));
$product->update_meta_data( 'wpneo_funding_maximum_price', intval($arr[0]));
$product->save();
}
}