Hi doctorini
You do realize your journal entry doesn't balance. I think you mean that Sales should be $10.
But if you want to show sales of $10 then you have to sell the item for $10. There is no getting around that.
There aren't too many ways of posting a discount to a separate GL account without keying in the discount separately. Douglas' method is one, but I think a better one (still not very good) is to use sales tax, assuming you aren't using sales tax for real. Adagio lets you enter a negative sales tax rate, the "tax" can go to any GL account you want, and it will calculate automatically.
If this appeals to you, but you are concerned that sales tax doesn't calculate until you actually ship an item, you can enter your orders in SalesCQ, which DOES calculate sales tax.
Steve