ActionScript 3.0 :: Drag And Drop Function - Bring Shape Back To Origin?

I have a relatively simple drag and drop function. When the target is hit, the shape stays on it. Otherwise, it goes back to its original place.

ActionScript Code:
square_mc.addEventListener(MouseEvent.MOUSE_DOWN, pickUp);
square_mc.addEventListener(MouseEvent.MOUSE_UP, dropIt);
function pickUp(event:MouseEvent):void {
event.target.startDrag(true);
event.target.parent.addChild(event.target);
[Code] .....

The slide function is supposed to bring the shape back to its origin (passx and passy are the coordinates), through an Enterframe event. It works fine, but it only works once. And if I drop the shape, if I pick it up again, it's impossible to "cancel" this action (which means the shape keeps going back to the origin). I tried removeEventListener but no luck.