The process has to start from a Purchase Order where all the items should be included. When you receive the items from the supplier, you shall choose Receive on Consignment (as the Document Template) and not Purchase Bill. That means you receive without a bill (invoice of your supplier).

When your supplier bills you for the items you've received then, you should open up the Purchase Order and you click Invoice. In the form that pops up, you include the quantity you have been invoiced for.

If you need to return back items to your supplier, you can create a Return to Supplier to send the received (and not sold items) back to your supplier. First you pick 'Return by Document Template and Document Number' and from the documents list, select Receive On Consignment and the number of the document you used to bring the products in. Then click "Ok" to create the Return to Supplier document.

After that, select the products you want to return. The system notifies you about the number of products that can be returned. You cannot return more quantity than the quantity of the related Document.