Scheduling Agreement

friends,

i have created a scheduling agreement for 100 quantity and i have maintained scheduled lines for it. ( 40 to be delivered this month, 40 in the next month and 20 after 3 months ). now when i am doing MIGO, i can see the total quantity of 100 in the feild target quantity, but when i enter the quantity received (i.e.40) it is showing the error message " PU quantity exceeded by 40 eaches....how this should be solved.

If anybody can guide me as how to ensure that MIGO is done with respect to the schedule lines. OR how to process schedule lines to receive the material from the vendor so that after receiving the material the schedule line for the quantity disappears from the screen MD04.

thanks