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 Goods Inbound (as the document type) and not Goods In and Sales Invoice. That means you receive without invoicing.
When you sell some products to a client, you open up the Purchase Order and you click Invoice. You include the items to invoice (the sold items only).
For the returns, you create a Return of Goods to Supplier to get the received (and not sold items) back to your supplier. From the documents list, select Goods In and the number of the document you used to bring the products in. Then click "Ok" to create the Return of Goods 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.