I need to produce an invoice and the serial number is not available, can I post is and edit the posted invoice?
No, if the sale has been processed then it is not able to be altered.
Process the sale with an incorrect serial number. Then credit and recharge with the correct serial number when this is available. (If you use Customer Items or OPE's the incorrect one will need to be deleted)
Save the invoice as Pending, print the invoice without the Serial number.