Hi ,
Please check whether the code that you have added is at the right place in the exit. It might so happen that the value you are trying to populate is getting overwritten or getting cleared some time later in the processing after executing your code.
Otherwise as suggested above, you can create a custom routine(copy of original with additional changes of your requirement ) in VOFM and try to assign it to conditions tab value with the help of your functional.
Regards,
Shruti
