Set discounts per specific draft orders

ie I need to display a different price to the product price in my invoice to a customer which is in a different currency. So for example - I'm selecting a product for the customer in the US and my store is UK based so GBP. I have quoted a certain USD price to my customer verbally. Now I need to raise an invoice for this new customer in USD which is fine but the discount requires a GBP price. I just want to set the final USD price for the order. IS this possible with your software?