inventory control

I want to monitor my inventory level every month. My type of business is buy and sell of inventory items. When a purchase order is received, I order the same to my supplier. However, there are times that I deliver a qty less than the order.
By way of example, a customer orders 12 pcs of item A but my on-hand qty is 10 pcs only, and so I deliver 10 pcs only but when I receive my order from my supplier, then I have to make another delivery to my customer. In this regard, I have two delivery receipts to my customer. How do I post in my quickbooks databse? Likewise, this is also similar to my supplier, if they have a staggered delivery based on 1 purchase order, how do I post it in my quickbooks database? Meaning to say, I have several delivery transactions in just one purchase order, either to my customer or from my supplier.

I hope you can assist me on this matter.

Thank you very much