[abelg]

Que tal, espero esto te ayude.

Código SQL [-]
CREATE FUNCTION dbo.Maximo_Ventas()
RETURNS @table_resultado TABLE (IdEmpleado int, Primer_Nombre nvarchar(10), Monto_total money)
AS
BEGIN
  DECLARE @TABLE TABLE (RowNumber int, IdEmpleado int, Primer_Nombre nvarchar(10), Monto_total money) 
  INSERT @TABLE
  SELECT ROW_NUMBER() OVER(ORDER BY ROUND(SUM((UnitPrice -(UnitPrice*Discount)) * Quantity), 2) desc) AS RowNumber, e.EmployeeID, e.FirstName, ROUND(SUM((UnitPrice -(UnitPrice*Discount)) * Quantity), 2) AS Total_Orden
  FROM Employees e
  INNER JOIN Orders o on o.EmployeeID = e.EmployeeID
  INNER JOIN [Order Details] od on od.OrderID = o.OrderID
  GROUP BY e.EmployeeID,
    e.FirstName

  INSERT INTO @table_resultado
  SELECT TOP 1 IdEmpleado, Primer_Nombre, Monto_total
  FROM @TABLE
  RETURN
END
GO

SELECT * FROM dbo.Maximo_Ventas()

Resultado
4 Margaret 232890.85

Saludos