If you have placed a purchase order and you have only received some items within your PO, you can mark this as partially received so only the products that have been sent to you by you supplier at that time will be added into your stock levels.
To do this please go to Inventory-> Purchase Orders-> Edit the relevant purchase order here.
Next please select 'PO Actions-> Receive
From here you can enter the quantity you have received and click 'receive'
The following page will appear, from here you can chose to 'keep the PO Open for the Remaining Items'
Your purchase order will then be displayed as 'completed_partial'
If you edit your purchase order you will be able to see the quantities received, and the remaining quantities to be shipped.