AR: Allow undoing writeoffs

It would be FANTASTIC if there was a way to undo a write off. Once you run the GL transfer as you know you cannot. We are paid by insurance companies and often the send payments months down the road for fees we wrote off so I then have to create a manual invoice to post the payment against which is a hassle and not great bookkeeping. So it would be great if you could undo or delete a write off.