/classes/Cart.php

PHP:
if ($cart_rule['obj']->reduction_percent || $cart_rule['obj']->reduction_amount 0)

меняем на

PHP:
if ($cart_rule['obj']->reduction_percent != || $cart_rule['obj']->reduction_amount 0)

/classes/CartRule.php

PHP:
'reduction_percent' =>         array('type' => self::TYPE_FLOAT'validate' => 'isPercentage'),

меняем на

PHP:
'reduction_percent' =>         array('type' => self::TYPE_FLOAT'validate' => 'isFloat'),

далее

PHP:
elseif ($cart_rule['reduction_percent'] > 0)

меняем на

PHP:
elseif ($cart_rule['reduction_percent'] > -100)

далее

PHP:
if ($this->reduction_percent && $this->reduction_product 0)

меняем на

PHP:
if ($this->reduction_percent && $this->reduction_product > -100)

/controllers/admin/AdminCartRulesController.php

PHP:
if ((float)Tools::getValue('reduction_percent') < || (float)Tools::getValue('reduction_percent') > 100)

меняем на

PHP:
if ((float)Tools::getValue('reduction_percent') < -100 || (float)Tools::getValue('reduction_percent') > 100)


/admin/themes/default/template/controllers/cart_rules/actions.tpl

Все строчки

PHP:
getFieldValue($currentObject'reduction_percent')|floatval 0

меняем на

PHP:
getFieldValue($currentObject'reduction_percent')|floatval > -100