I had the same thing happen with a platy I wanted. It was a beautiful red velvet platy with a black tail, but red top fin.
He jumped to the concrete floor from a small container while the guy at the LFS was catching another fish. It took about 30 seconds to find the little guy. I brought him home anyway.
After a few days he developed fin rot on his tail and two side fins. It got pretty bad, but after some Melafix, he started to get better.
His fins got better, but he never really recovered from his ordeal. He is still alive, but very timid and spends most of his time hiding behind plants.
He doesn't swim very well, kinda zips all over the place, and that's only when there is food around.
I feel sorry for the little guy.