In order to have the option to select a warehouse in the order whilst taking into account the amount of stock leftovers in it, you must enable the “Reservation of stock balances” option in the Warehouses.
In this case, products for which no warehouse has been selected in the order will have the “No reservation” text. When you click on it, a form will open where you can specify a warehouse for each single product, taking into account the current stock balances. Only those warehouses with a minimum of 1 item of stock are displayed. Warehouses with the "Availability" stock balances type are displayed only when the "Yes" option is selected for them.
For each product in this form, you can specify the quantity for a specific warehouse. The system will not allow you to specify a higher quantity of products than are currently available.
If fewer items have been reserved in the warehouses than specified in the order, a system message will appear notifying how many units have not yet been reserved.
Note
For warehouses of the "Supplier warehouse" type with the "Availability" type of stock balance (yes / no), or with active integration with an external warehouse system, free stock balances are not written off when reserving.