Hi Lian
Never say Die!! 
your case corresponds to what is there in note 137552.. It populates Customer no in the discount line item automatically... See if this helps you
Note that all the Org elements like Profit center, Busn Area etc are auto populated in PSG to maintain consistency.. With this note, you can also copy the Customer No. etc
This note uses exit COPA0005.. You can write your own code in COPA0005 incase this note does not help
Br, Ajay M