Item category - Relevant for billing 'G'

hello!

i want to invoice based on sales order, but only the delivered quantity. due to process limitation i do not want to use delivery related billing.

did anyone use scenario like that before? when i create an invoice for a sales order which has not been delivered yet, the system proposed the full quantity of the sales order. do i need to use some special formula during copy from sales order->invoice?

thanks in advance!

matthias