Assuming Inv_override is entered after the Transfer macro is run,
and assuming 'override B16' means over write B16,
then you should be able to use the Worksheet_Change event.
Try this in the "Order" sheet module
Code:
Private Sub Worksheet_Change(ByVal Target As Range)
'limit to single cell
If Target.Count > 1 Or Target.address <> "$B$17" Then Exit Sub
' value in b17 has changed to something other than blank
If Target.Value <> "" Then
With Sheets("Order")
.Unprotect
Application.EnableEvents = False
.Range("B16").Value = Target.Value
Application.EnableEvents = True
.Protect
End With
End If
End Sub