Dear kiran ,
as per lakshmipathi given solution to u r issue .i think u r are facing any other issue kindly let me know .
i will also give some more information regarding to your issue .if you want to reshudule date with ATP check means , double click on item and click on sechudule tab .change the date and it automatcally ask total 100 qty for ATP check or if you cant able to do ATP check due to any validation, change the line item qty to 50 and create one more line item with another 50 qty .
Regards
C.B Reddy
