Your post implies that only the finish was damaged and not the underlying wood. If so, all you're trying to do is build up enough of a coating to match the undamaged portion.

I've used high gloss polyurethane on one of my panels with pretty good results.

I'd definitely remove the panel before working on it. (You'll get much better results if you lay the wood out flat--on a level surface.)

Obviously, make sure the area where you work on the wood is spotless: dust and other debris are real no-no's. Apply a thin coat of the polyurethane (I used Minwax(TM) brand, but I suspect it doesn't matter) to the damaged corner using a fresh brush (those little touch-up brushes work pretty well.)

Let it dry overnight.

When dry, with a very fine grit sandpaper, carefully rub the repaired area LIGHTLY to prepare the surface for the next coat. Clean away the resulting dust with a damp, lint-free cloth; dry; and apply the next coat of the polyurethane.

Two coats may be all you need, but if the repaired corner is still not level with the surrounding surfaces, then repeat the process until you're satisfied.

Remember: "Patience is a virtue" here! Don't rush the process by "glomming-on" too much polyurethane at once. Take your time and I think you'll be reasonably satisfied with the outcome.

Good luck. And, let us know how it goes...

__________________
Eric

1991 560SEL-Euro (214,000 miles)
1989 420SEL (Retired from daily use at 325,000 miles; Use as donor vehicle)