Purchase Order = items you have ordered to increase your inventory? That seems unusual that you would immediately increase your inventory at the time you place such an order unless you have very fast, very reliable vendors supplying that product. But maybe you are logging a decrease in inventory due to selling product to a customer (which is usually not called a "purchase order" but rather an "order" or "invoice".)
See this thread for a discussion of how to use the "line items" part of a typical invoicing system to log immediate inventory changes. A similar set up with Purchase Orders could also log increases in inventory.