Hello SAGEX, 

You are managing serial numbers only when issuing the stock.

In this case they are not managed in stock.

You can view the issued serial numbers under Serial number inquiries.

When validating a sales delivery, system will automatically generate the starting and ending serial numbers based on the sequence number assigned on the product serial number management setup. Or you have the option to manually enter then as well. This can be done in the modification of issues screen. Clear the stock line selection and click refresh to pick the stock line and then enter the serial numbers manually.

Issued serial numbers are not managed on the sales order level. 

If the sales order is a direct invoice then you can enter the serial number at the invoice level.