AFS deallocation

Hello,

I have a requirement where for the order type RUSH , when there is a particular delivery block 'XY' on the header or grid level ,the allocation should not be done.I have acheived this using the user exit for ARUN . But the problem now is when I enter the grid data first and later the delivery block on the header the allocation is already done as soon as I entered the grid data . So, I would like to deallocate the quantity as soon as the delivery block is entered. Could anyone of you please suggest a function module where I could do the deallocation using it ?

Thanks

Aruna