No. The only way to keep the sales order open would be to change the quantity. In your case you may be able to apply the quantity of .4. .4 of 1 would be 2000.
You could change the sales order as follows. When charging labor it is usually a unit of measure of hours. So you would want to set up your unit pricing as x/hour then apply the quantity. Every time you deliver x number of hours then you would change the quantity to apply that amount of hours. This would leave the additional hours open.
