Have you tried the Quote functions? You can enter a quote like a sale and print it out. But it doesn't accept a down payment.
You could enter a unattached payment from the customer and leave the sale open. With the Customer Credit function on, later add the final sale amount and Finish the sale. The credit amount would be applied to the sale. This type of transaction is not directly supported. You would have to do some kind of work around like this.
