How to Create Dynamic SQL Connection String in SSIS

Hi Everybody,
I have created a very simple SSIS Package which gets the sql records and put into Excel file. It is working fine in my development server with window authentication. But I want to use sql authentication in my sql connection. So I am passing variables (password and server)  to sql connection string in expression property like following:
"Data Source=" + @[User::Server] + ";User ID=adaptuser;Password=" + @[User::Password]  + ";Initial Catalog=ADAPT;Provider=SQLNCLI.1;Integrated Security=SSPI;Auto Translate=False;"

And I am calling this package from my stored procedure. The problem is it is not overwriting the password value in the configuration even If I put wrong password still it is working which means it is not taking values from variable. But if i change the Server variable value to wrong values then it gives me error. Now can someone explain me or tell how to force package to use password value from variable because I can not edit package everytime while deplying this package on the live server. I am using the following code in the stored procedure to run the package:
Declare @Cmd varchar(5000), @returncode int
Select @Cmd = 'dtexec /F "' + @SSIPath + '"'
Select @Cmd = @Cmd + ' /SET \Package.Variables[User::Server].Properties[Value];"GfKAUDEV1"'
Select @Cmd = @Cmd + ' /SET \Package.Variables[User::Password].Properties[Value];"temporary"'
EXEC @returncode = xp_cmdshell @Cmd
Select @returncode
I hope above explain enough that what is the problem.
Please help me on this issue.