Editing Project Invoices

Hi,
We are doing Work based billing. We have a lot of inter-project invoices. We are putting a markup on the cost rate of the expenditure types to generate the invoices. Our Financial controller, tend to miss some transactions due to which the inter-project invoices generated on monthend are not of the correct amount. We want to have the flexibility of editing the invoices, so that the bill amount can be adjusted, before they are interfaced to AR. Please let me know if this is possible.

Regards,