nehancements

hi i am doing an enhancements in which in va01 transaction whenever the user does any change to the pricing conditions(reduction) then a pop up should come to select a reason code for the reduction. this popup shoul dcome from the ztable that i have created. how to do the code for this!! n also this should come only once for the change to the current transaction.if there is further change in the surrent transaction then this popup should not come!! can u plss help me with the code pls!!

POINTS PROMISED!!THANX IN ADVANCE!!