If you're in a situation where you've received products, but for whatever reason, need to ship them back to the the supplier and remove them from your inventory, the best way to do this is to create a RTV (Return to Vendor) or RTS (Return to Supplier) purchase order.
Step 1: Create a purchase order. It can be helpful to use the original purchase order and add the suffix "-RTV" to the end of it so you can keep track that you're returning products from that order. This is optional, you can name it however you'd like.
Step 2: Add the products you are returning to the supplier.
Step 3: Add the quantities that you are shipping back, but in a negative amount. If you're sending back 10, the quantity needs to be -10.
Step 4: Process the shipment using negative quantities for the amounts as well. Once you "receive" the shipment, it will pull the stock from your inventory.