HELP! PLEASE! Something is wrong with my bird!?

He is a zebra finch and his right leg is really really weird! It's all swollen and deformed and a weird dark color. My mom said he probably got cut and he will get better on his own. I told her I don't want to risk his life! I know birds are really little and he can easily die of an infection. I am really scared! I told her I will use my money from my sweet 15 party, but she said I shouldn't waste it. I really don't care how much of my money I use. I love my bird so much! Could it really just be a cut? I think there was some blood, but I'm worried because there's black stuff around his foot. It hurts him because he can't walk right and he sometimes walks on just one leg. I noticed it just yesterday. Please help me!