At month-end when the customer is invoiced, we need to adjust the net amount
Already given suggestion will work only if it is invoiced then and there. From your comments, I presume, invoice would be generated regularly in a month and at the month end, you need to adjust the value to the extent of quantity lifted.
If my understanding is correct, then I would recommend you to go with Credit Memo functionality which is simple, straight and can be achieved with standard configuration
G. Lakshmipathi