SSIS DTEXEC Date Variables

I have a SSIS package will am executing by calling a store procedure. The script of the store procedure
as below

ALTER PROCEDURE [dbo].[Generate_TrialBalance]

@TBDate varchar(50)

AS
BEGIN

DECLARE @dtsExecCmd varchar(4000)
SET @dtsExecCmd = @dtsExecCmd + ' /SET "\Package.Variables[User::TBDate].Properties
[Value]";' + @TBDate

EXEC master..xp_cmdshell @dtsExecCmd

END

When i Execute the store procedure like EXEC [Generate_TrialBalance] '01/01/2013' it returns no error except NULL in the result pane.
Where am i going wrong. Note that the ssis package has a package level variable called TBDate mapped appropriately to a parameter in my oledbsource query parameter.

Did you print out @dtsExecCmd and double check? Does it work in dos prompt?