CustomBooks™ has powerful multi-location features that can handle track the inventory for drop shipping. Here's one possible solution:
1.) Enable Multi-location feature in Admin Panel → Inventory Settings.
2.) Create a location called Dropship.
3.) When creating the PO, use Dropship as the location.
The dropship location is a virtual "location" rather than a physical one that would "hold" the inventory from the time you ordered it from the drop shipper until the time you invoice your customer.