If you followed the customaziation for adding negative discount in this question https://magento.stackexchange.com/a/261752/76597 there should be correct price entries in the table catalogrule_product_price
for the corresponding products.
-- change the values with those of your rule / prodcut you want to check and the actual date
select * from catalogrule_product_price where rule_date = '2019-02-15' and customer_group_id = 0 and product_id = 44;
If you find that entry your rule is working.
There is only one more hint: In Mage_CatalogRule_Model_Observer
there are two checks where the rule price is only applied, if it is below the regular price. Therefore you have to overwrite the two methods processFrontFinalPrice($observer)
and processAdminFinalPrice($observer)
and replace and bypass that check. For example like this:
if ($this->_rulePrices[$key]!==false) {
// $finalPrice = min($product->getData('final_price'), $this->_rulePrices[$key]);
$finalPrice = $this->_rulePrices[$key];
$product->setFinalPrice($finalPrice);
}
I hope that helps.