Resignation Letter
My understanding was that the reason that they need the resignation letter was so that they could process the UAE residence visa, as for EK to be your sponsor in the country you cannot be employed by anyone else.
When I got my letter, I asked to have it done on the company letterhead, and they stamped it for me, it just said how long I had worked there and when I terminated my employment. But then again I was a casual so didn't have the worry of a payout over my head.
Hope this helps