Thanks for your support of OP to add an internal auditor to the staff. 

Another task for such internal auditor is to review any "extras" being billed by contractors, beyond the initially agreed contractual price, prior to payment.  That area of billings for "extras" is fertile ground for contractors to pad their profits.  Needs plenty of scrutiny.  Also estimates approved in advance, etc. etc.

PS you may not be surprised that part of my working career I was an internal auditor.

Tom