Pre-Order Manager allows you to switch your items automatically to pre-order if they purchased more than the available quantities. Pre-Order Manager will promptly notify the customers if part of their order is under pre-order upon adding the items to cart.
To modify the notification message, go to "Settings > App Settings" and change the value under "Part of the order will be preordered".
Partial Pre-Orders in the Shopping Cart
If a customer had purchased more than the available inventories for a pre-order enabled product, this is how it is reflected in their cart: