Tax on Pre-discount Amount

We have a scenario where a customer will prepay 25% of an order before the order ships and then pay the balance according to the payment terms on the invoice. We want the 25% prepayment to show on the invoice as a discount; however, we also want the taxes calculated on the pre-discount/gross amount. For example if the pre-discount/unit price = $1000 and the tax rate is 5%, then the invoice should show a line for $1000 less $250 Prepayment Discount plus $50 tax (5% of $1000, not 5% of $750). What is the best way to configure this?

Our SR response was "there is no way to accomplish the scenario as described".  Anyone have any ideas on how this can be addressed?