Best Practices for Creating Time and Material Purchase Orders

What options are there in creating Purchase Orders for unknown quantities of Time and Material?  Our users often create the T&M PO after the invoice has been received, using the actual amounts from the invoice.  We want them to have the PO entered and approved before we receive and enter the invoice.  I would like to have a couple of ideas to present to our purchasing department, showing them ways in which this type of PO can be created.

We are using Oracle EBS 12.1.3