Getting Final Discount Amount from Shopping Cart Promotion

I'm trying to add Final Promotion Discount Amount to my Amazon Payment module in Cba.php

    $_xml .= " <Promotions>\n"
            ."    <Promotion>\n"
            ."      <PromotionId>cart-total-discount</PromotionId>\n"
            ."      <Description>Coupon</Description>\n"
            ."      <Benefit>\n"
            ."         <FixedAmountDiscount>\n"
            ."           <Amount>".  ."</Amount>\n"
            ."         </FixedAmountDiscount>\n"
            ."      </Benefit>\n"
            ."    </Promoti开发者_开发技巧on>\n"
            ."  </Promotions>\n";

I need to add the final discount amount from Shopping Cart Promotion Rule for the Amount tag.

Does anyone know how I can access this amount?

all amounts you can access from order or quote you can debug/reveal as follows

print_r($order->getData());
print_r($quote->getData());

although it is suggested that you use debugger like xdebug or zend to see what is inside objects and variables at your breakpoint.