Monet is lovely, I like the blend of colors. I want to clarify, it's his dorsal fin, right? It looks like it's broken and bent over on itself? Hard to see because the round cup distorts the view.
If he has trouble swimming and he's calm enough, I'd suggest keeping him in his cup for now and floating it in a big, warm tank. Make a mixture of 1 tsp of aquarium salt per gallon and use that for him. Change the water every day, redo the salt, keep him like that until he goes stir crazy and wants out. A product like Prime or Stress Coat as water conditioner will help too.
If you don't want to keep him in the cup, then lower the water levels a little of his tank so he can reach the surface easier. As long as you don't have live plants, you can do the salt treatment in his big tank, too (it's just easier to change the water daily on a small QT cup). If you do have live plants, you'll have to use the cup as salt can kill plants.
You may find the broken part of his fin falls off in time. In fact, it might be better for him if it did. If it does, keep the water super clean and continue with the salt to keep away secondary infections.