Manual Billing document not deleting

All,

I have encountered one issue and the details for the same are. There is one open Final billing order which users wanted to close but with zero amount and they wanted to move in new BP into the same installation and then start billing. so They created one manual billing document with zero amount but    didn't select the billing order while creating the billing document and invoiced it. Since they had to create billing document against the Final billing order, so the user tried to reverse the Invoice as well as billing document using transaction EA15 which has reversed the invoice as well as manual billing doc.

Now the further billing is not allowed against the billing order as system is giving error that already billing document exists for the same period.

Now the deletion of that manual billing document is not happening as system is showing that Document is already used.

I can see that billing order is billable but still getting this error.

And heads up about this scenario will be helpful.

Thanks,

Tripti