MZI - 2006-05-22

I am testing the prepayment functionality. And I didnt find in manual(253a) or forum if this is posible in standart Compiere.  

1)I want to create prepay order, but only for 50% of value that will be in invoice. So after automatic creation of invoice I will have open invoice for the rest 50%.

2)In prepay order is let say 30Kg of material(6pcs). After I receive prepayment I do delivery, but this 6 pcs will be 31.35Kg. The price in contract is set for Kg so I have to use kg. And I know exact weight only after shipment. Is there any way to change Invoice and add line with extra weight and the open ammount for invoice will be only the value of new line?

Pls help,
Thanks MisoZ