Calcular la suma de valores en el árbol (consulta recursiva)

Tu intento de usar LEAD no funcionará porque no está sumando todos los niveles anteriores y sus ID deben ser secuenciales.

Desglose la jerarquía completa para cada empleado primero de modo que cada empleado se incluya una vez por nivel de jerarquía:

;WITH cte 
AS
(
  SELECT e.ID, e.Name, e.ID as sub_ID 
  FROM @Employees e
  -- no WHERE-condition to get all employees
  UNION ALL
  SELECT 
     c.ID, c.Name -- keep the initial employee
     ,e.ID as sub_ID
  FROM @Employees e
    INNER JOIN cte c ON c.sub_ID = e.ParentID
)

SELECT 
     c.ID
    ,c.Name
    -- parent level
    ,sum(case when c.id =  s.EmployeeID then s.Quantity else 0 end) AS ParentSumSales
    -- child level
    ,sum(case when c.id <> s.EmployeeID then s.Quantity else 0 end) AS ChildSumSales
FROM cte c
LEFT JOIN @Sales as s
ON s.EmployeeID = c.sub_ID
group by c.Name, c.id