Hi,
Thank you for your feedback.
Change the line:
$query = 'UPDATE '.hikashop_table('discount').' SET discount_used_times = discount_used_times - 1 WHERE discount_code='.$this->database->Quote($code).' AND discount_type=\'coupon\' LIMIT 1';
to:
$query = 'UPDATE '.hikashop_table('discount').' SET discount_used_times = discount_used_times - 1 WHERE discount_used_times> 0 AND discount_code='.$this->database->Quote($code).' AND discount_type=\'coupon\' LIMIT 1';
in the file administrator/components/com_hikashop/classes/order.php and it will fix the problem.
We'll add the fix on our end for the next version of HikaShop which is scheduled for the next few days.