When you ship the items to the client, you shall choose Ship On Consigment (as the Document Template) and not the Sales Invoice. That means you ship without invoicing.

When your client sells to the end client, you open up the Sales Order and you click Consigment Invoice. You include the items to invoice (the sold items only).

For the returns, you create a Return of Goods from Client to get the shipped (and not sold items) back to your warehouse. 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.