Set up a calculation field similar to:
If ( Position = "manager" ; Employee Name )
This field will be empty if the employee does not have "manager" in the position field. Base your value list on this field instead of the employee Name field. Your value list can list employeeID as field 1 and this calcualtion field as field 2 if you specify that only the second field should be shown in the value list.
So simple... Thank you Phil!