How to Apply Discounts on Unearned/Deferred Revenues

We have a product we are selling as a pre-order. To accomplish this, first we set up two items: "Product A" and "Product A Pre-Order". Product A is setup to go the "Product A Sales" revenue account and Product A Pre-Order is setup to go the "Unearned Revenue" liability account under the revenue account. In addition to the two product items, we have 1 "Discount" misc. charge setup that mapped to the "Discounts" G/L account.

Now, to enter in the pre order, we added a Sales Receipt using the "Product A Pre-Order" item as the product (again, this will map this to the "Unearned Revenue" account).

My question is how do we apply a Discount to that same transaction?

If we use the Discount item, it will record a Discount on the P&L at the time of the Sales Receipt (or pre-order) and that is incorrect. It should only apply the Discount at the time of invoicing later on when we deliver the product.