I don't recall mine being leather, but what I do recall, is that the material was loosearound that rectangle, and that it looked like it had shrunk, so even gluing it back on there, would have left unsightly exposed areas of metal.
Fortunately for me, the dealer I bought it from said that they would pay to have both sides replaced. I ended up taking the car to the repair place myself. He ended up removing the entire post, and putting on new material. It wasn't leather, but then again, nor was the old material. Unfortunately, I do not remember what he used for an adhesive, but I am thinking it was a 3M product. The only difference in material that I noted, was that the original had a smidgeon of foam backing.
It probably took him about 10 minutes to remove each side, and about the same to replace the material, and about the same to put each side back into place.
"...rationality of thought imposes a limit on a person's concept of his relation to the cosmos." -Nash
04/71 280SE 195k
09 Jetta TDI