Availability check for sales order

Hi,

In our sales order, we create automatically a purchase requisition.

Our customer need to create a purchase order after checking stocks, and create a purchase order only for the quantity remaining in stock.

For example, if we have 10 UN in stock, and the customer order 20, the system must reserve 10 UN to this order and create automatically a purchase order for 10 remains.

it's possible ?

Thanks for your help.

best regards,