You can add discounts to a particular product by using the default Magento feature of Product Tier Pricing.
Refer the link for the detailed information:
You need to create a Cart Price Rule select Apply="Fixed Amount Discount" and within the "Apply the rule only to cart items matching the following conditions" - section you have to select the Product A e.g. by providing the SKU.
To have the SKU available here, you may activate first "Use for Promo Rule Conditions" within the ...