Fur fungus can take on a variety of different forms depending on the bug that is infectign the animal. Either way, you know your chin is sick and you do not know what it is sick with. While we can provide guesses, we are not medical professionals. That would be why a vet visit is advisable.
No offence, but if all you can afford is the chinchilla itself and cannot afford to provide it medical care, maybe you should reconsider even owning one. It is really not doing your animal justice to just let it get sick and not provide care based on lack of money. Medical needs are always an issue at some point in a pet's life and there should be a plan in place to provide what is necessary even before the animal steps into your house. It's just not enough to want a pet for companionship or any other reason. You need to take care of it when required too.
