Downpayment or Deposit Invoices Qty's

Hi,

I want to start using the downpayment/deposit invoice option of Dolibarr. During the tests I conducted, the downpayment is calculated by adjusting the qty of the items. Ex.: If I have 10 pcs. if a certain item and I generate a downpayment invoice of 40%, the qty is adjusted to 4 pcs. instead of 10 pcs. Is there any way to calculate the downpayment amount from the final amount instead of adjusting the qty’s of the items? Any help would be appreciated.