Dropsy is extremely difficult to cure. I would call the pet shop and ask them whether or not they allow customer to exchange sick fish for a healthy one.  Dropsy is often caused by poor water quality (high Ammonia , Nitrate (NO3-) and Nitrite (NO2) levels), which in this case is poor water maintenance at the pet shop or at the place where the pet shop acquired these fish. Most likely the fish will never be normal again if it manages to survive.