Buy one item and get another item at a discounted price

Summary: Business scenario: Our client has a promotional campaign where they offer an item Y at a specific dicounted price when their customer buys an item X. The items X and Y are not fixed. Pricing can't handled such a scenario.

Content (required):

Customer places an order of item X. This is entered in the sales order line 1. The requirement is that item Y is added on order line 2 automatically and at a discounted price.

Addition of item Y can be achieved by Product Transformation Rules but the problem is that this line comes only with 0 price and doesn't meet the requirement.