Dual-Level Authorisation

We would like to use the Purchase Order function more extensively in our company, but need two authorisation levels per PO.  So, as an example, that would mean our HODs can authorise up to £999, above £999 our CFO has to authorise.  I've needed this function in so many companies I've worked at, strangely we always seem to go back to printed books, nothing else has quite the flexibility and we normally need to buy too many user licences.  Now that we need more people involved in the inventory functions it makes sense to deploy DEAR for POs too.  Thank you