Set fixed total price for a sales order

Allow user to select a fixed price for a sales order/quotation/pro-forma/return. As secondary to this; give user disbursement options so that tax (if using multiple tax codes) and profitability reports are maintained This is regularly requested by clients. The best options available at present are to add a free text line with a negative value; or workout the discount percentage (%) and apply to the order or each line on the order