You can make the settings for the billing quantity in the copy control. You have the options of keeping the billing quantity equal to order quantity or delivery quantity or delivered quantity minus the quantity already invoiced. The billing quantity indicator specifies which quantity the system copies from the source document (a sales order, for example) into the target billing document (an invoice, for example).
Make sure that control of the amount to be billed is directly related to the item category billing relevance. (e.g. K – delivery related invoice for partial quantity, G – Order related billing of the delivery quantity etc.)