On behalf of SAP responsibility, If you need to issue the invoice with old material (stock avaiable), you just do
1) MM will check status in 'Sales: sales org.1' , have to be 'SA'
Status 'SA' means can do only the invoice but can't do the issue PO and start production
2) Please check the procurement type in Sale Order, It have to be 'KL' (It will take from Stock Available)
Then, you can do the invoice.