Links one of the caller's purchase orders to one of its sales orders: the purchase order's supplier side is reconnected to the sales order's buyer node. Repeating a link is harmless. 404 not_found when either order is missing, removed, archived or another company's; 409 cannot_link_order_with_different_attachment_template; 400 cannot_link_orders_with_same_supplier_and_buyer_company_of_the_initial_order when the link would loop back to the company the sales order already supplies. The chain, the link and the partners' notifications are refreshed in the background.