Allow to change picking amount during pick

We have lot of boxes of inserts which are not full (returned after jobs). So when we pick these items invetory shows for example 30 pieces (full box is 10 pieces). In real life there might be for example five boxes of different amounts. If person could change picking amount while locker is open it would greatly ease our picking. Now there is only two ways to do it "right":

1. Cancel the pick and start from beginning with the amount corresponding some box.

2. Collect original amount from several boxes.