paypal discount amount

hello,

I wonder if anyone can shed some light on a problem Im having,
paypal seems not to want to take a discount amount when Im passing the variables over,
takes the discount_rate_cart variable but I cant seem to get it to accept the discount_amount_cart and I cant see any problem.

Code:

$config['discount_amount_cart'] = 2.99; //Discount amount [2.99]
        $config["discount_rate_cart"]     = 5; //Discount percentage [5]

percentage rate works fine but the cart amount discount wont pass over, anyone know wat Im missing?

any help would be great

[edit to add]
Im using this paypal library for codeigniter
/**
* Paypal Class
*
* @package CodeIgniter
* @subpackage Libraries
* @category Payment process
* @author Romaldy Minaya
*