In order to avoid having your stock reserved when you confirm the order, the route Replenish to order (MTO) has to be used. This route is initially archived in the database. (routes>filter on archived).

Check that the route is applicable on sales order lines :

In the sale order, click on the 3 dots and check the case "route"

All the products with the MTO route set will generate a purchase order (one per supplier concerned) or a manufacturing order depending on the product.

Important element:

Initially, the route is set as "Take from stock, if not enough available, trigger another rule". This means that if you have enough stock, it will be reserved and no PO / MO will be generated. If you don't have enough stock available, the rule for generating the PO / MO will be triggered.

In this case, as we never want the stock to be reserved we don't want this the "take from stock" part to happen, only the "trigger another rule".

Therefore, we need to modify the route (more specifically the rules within the route

=> Inventory > Routes > Replenishment on order (MTO), and click on the rules down in the form. 

=> change the supply method from "Take from stock, if unavailable, Trigger another rule" to "Trigger another rule".

If you have a simple PO => SO flow, just modify the route from Stock location to Partner locations / Customers as destination. 

If there is an MO to be generated, the same change can be applied to the components (Stock to Virtual locations / production).

Eventually, when create a sale order we the MTO rule selected. A PO/MO will be generated but no quantities will be reserved on my delivery slip, allowing me to keep working with my MTS orders. 

Once the products from the generated PO / MO have been receipt / produced, the quantity will automatically reserved on my delivery slip.