At the moment the only type of discount available through that "cart discount" method is to do it by percentage, but you can apply a discount by amount to each item.

Simply add the desired product to cart, then click on the product name to open the item. From there, you can select Add Discount in the lower right corner:

Here, you can input the discount value and select whether you would like it applied as a percentage or deducted as an amount from the item price:

After applying the item-level discount, you should see the original price crossed off in the cart as well as the new discounted price: