Using accumulative sum on ordered MDX query

I have an MDX query That is sorted and I want to add an accumulative sum column.

The problem is that when I use an aggregate function, it still sum the values in default order, not the order I specified. below is my MDX query:

WITH 
  MEMBER RunningTotal AS 
    Aggregate
    (
      {
          [Dim Product].[ProductName].FirstChild
        : 
          [Dim Product].[ProductName].CurrentMember
      }
     ,[Measures].[Sales Amount]
    ) 
SELECT 
  {
    [Measures].[Sales Amount]
   ,RunningTotal
  } ON 0
 ,NonEmpty
  (
    Order
    (
      [Dim Product].[ProductName].[ProductName]
     ,[Measures].[Sales Amount]
     ,Desc
    )
  ) ON 1
FROM [InternetSales];

This is your query - you mention a 'specified order' but I cannot see one:

WITH 
  MEMBER RunningTotal AS 
    Aggregate
    (
      {
          [Dim Product].[ProductName].FirstChild
        : 
          [Dim Product].[ProductName].CurrentMember
      }
     ,[Measures].[Sales Amount]
    ) 
SELECT 
  {
    [Measures].[Sales Amount]
   ,RunningTotal
  } ON COLUMNS
 ,NON EMPTY 
    [Dim Product].[ProductName].[ProductName] ON ROWS
FROM [InternetSales];

As an example I think if you'd like to order the rows by Sales Amount then you could introduce an ORDER function:

WITH 
  MEMBER RunningTotal AS 
    Aggregate
    (
      {
          [Dim Product].[ProductName].FirstChild
        : 
          [Dim Product].[ProductName].CurrentMember
      }
     ,[Measures].[Sales Amount]
    ) 
SELECT 
  {
    [Measures].[Sales Amount]
   ,RunningTotal
  } ON 0
 ,NON EMPTY 
    Order
    (
      [Dim Product].[ProductName].[ProductName]
     ,[Measures].[Sales Amount]
     ,BDESC
    ) ON 1
FROM [InternetSales];

Example against AdvWrks:

WITH 
  MEMBER RunningTotal AS 
    Aggregate
    (
      {
          [Product].[Category].FirstChild
        : 
          [Product].[Category].CurrentMember
      }
     ,[Measures].[Sales Amount]
    ) 
SELECT 
  {
    [Measures].[Sales Amount]
   ,RunningTotal
  } ON 0
 ,NON EMPTY 
    Order
    (
      [Product].[Category].[Category]
     ,[Measures].[RunningTotal]
     ,BASC
    ) ON 1
FROM [Adventure Works]

Returns the following:

An alternative that gets around the ordering inherited from the cube is the following:

WITH 
  SET X_UNORDERED AS 
    {
      NonEmpty
      (
        {[Product].[Category].[Category].MEMBERS}
       ,[Measures].[Sales Amount]
      )
    } 
  SET X_ORDERED AS 
    {Order(X_UNORDERED,[Measures].[Sales Amount])} 
  MEMBER [Measures].[Rank] AS 
    Rank
    (
      [Product].[Category].CurrentMember
     ,X_ORDERED
    ) 
  MEMBER [Measures].[RunningSum] AS 
    IIF
    (
      [Measures].[Rank] = 1
     ,[Measures].[Sales Amount]
     ,
        [Measures].[Sales Amount]
      + 
        (
          [Measures].[RunningSum]
         ,X_ORDERED.Item(
          [Measures].[Rank] - 2)
        )
    ), FORMAT_STRING="$#,#.00"
SELECT 
  {
    [Measures].[Sales Amount]
   ,[Measures].[RunningSum]
  } ON 0
 ,X_ORDERED ON 1
FROM [Adventure Works];

Returns the values in the order you want: