Does anyone know if OpenAir has the ability to create a credit note to reverse an invoice?

We have a situation where we invoiced a client last year for time charges. The charges exceed the balance remaining on a purchase order, so the client has asked for the invoice to be re-issued as 2 seperate invoices to fit within 2 seperate purchase orders.

As the invoice was issued last year, the revenue has been finalised and reported, so we need reverse and re-issue the new invoices for the same value in a way that would re-open up the time charges but not remove the revenue already reported.