Q: "Automatically create/update an invoice on entering a bill".
I have this option unticked yet the invoice updates automatically when the bill was entered and differed
A: Invoices will always update once they know the correct Actual cost.
Q: Unable to delete an invoice, Getting an error
"This invoice cannot be deleted because it's part of a ledger transaction".
A: Once the invoice has been linked to a transaction it may not be deleted as it is part of the audit trail.