Dynamic SQL!

Hi there,
I am trying to create a dynamic sql statement as follows:
ALTER PROCEDURE [dbo].[GET_FIS_LONGTITLE]
    -- Add the parameters for the stored procedure here
    @TABLENAME VARCHAR(25),
    @COLUMNNAME VARCHAR(25),
    @COLUMNVALUE VARCHAR(25),
    @RETURNVALUE VARCHAR(60) OUT
AS
BEGIN

    DECLARE @SQL AS VARCHAR(4000)

    SET @SQL = 'SELECT LONGTITLE FROM ' + CAST(@TABLENAME AS VARCHAR(25)) +
            ' WHERE ' + CAST(@COLUMNNAME AS VARCHAR(25)) + ' = ''' + CAST(@COLUMNVALUE AS VARCHAR(25)) + ''''
   
    execute (@SQL)   

    --''' + CAST(@RETURNVALUE AS VARCHAR(60)) + ''' =
   
END
 
here I am trying to get the long title of an item based on the tablename, columnname, the column value. So the select returns the long title from the table as required.
But I want to assign that value to the @RETURNVALUE So I tried:
SET @SQL = 'SELECT ''' + CAST(@RETURNVALUE AS VARCHAR(60)) + '''  = LONGTITLE FROM ' + CAST(@TABLENAME AS VARCHAR(25)) +

            ' WHERE ' + CAST(@COLUMNNAME AS VARCHAR(25)) + ' = ''' + CAST(@COLUMNVALUE AS VARCHAR(25)) + '''' 

It does not work. I do not know what is missing here.
Any help would be greatly appreciated.
thanks,
Murthy here