FooEvents POS supports the native backorder functionality in WooCommerce.
In order to allow products to be sold on backorder, first ensure that you have enabled “Enable stock management” in WooCommerce at a global level (WooCommerce > Settings > Products > Inventory).
Next, check that “Enable stock management at product level” is also set for each product that you would like to support backorders in the ‘Inventory’ product data tab. Finally, ensure that the “Allow backorders?” drop-down is set to either “Allow” or “Allow, but notify customer“.
You will be able to place orders for products that are on backorder in FooEvents POS if everything is configured correctly.