Hi,
1 - I'm afraid that's not related to our plugin or to our support.
2 - In the file "administrator/components/com_hikashop/classes/order_product.php", please replace
$query = 'UPDATE '.hikashop_table('product').' SET product_sales = GREATEST(0, product_sales - '.(int)$k.') WHERE product_id IN ('.implode(',',$update).')';
By
$query = 'UPDATE '.hikashop_table('product').' SET product_sales = GREATEST(0, CAST(product_sales AS SIGNED) - '.(int)$k.') WHERE product_id IN ('.implode(',',$update).')';
It should avoid the mysql bug regarding the operation made on an unsigned even if the greatest should avoid it.
Regards,