My hamster has a brown lump in the middle of his stomach!?!?

My hamster is a 16 month old dwarf hamster male. I was on vacation for two weeks, and my aunt was taking care of him. I just brought him home yesterday, and I had him in the play pen and I noticed when he rolled around in his bedding, a greenish brown lump on him stomach. I put him in his ball and held it over my head, and indeed and long skinny greenish brown bump on his stomach. I scruffed him and tried to get an even better look, but he was squirming too much. I didn't want to try touching it because I don't know what it is I don't want to possibly infect it. I showed it to my mom, and she said it was food or poop tangled in his fur, so I tried touching it gently and it wouldn't come out. My mom said it was really dirty and stuck in there and it would be gone by the next day. First thing I did today, check on the lump. Still there. Now, my concern is he might have a cancer tumor. :( But I thought cancer would change behaviour but he is still rowdy and hyper like he always is. He eats often, drinks daily, poops and pees like normal, is friendly, still has the most glossy, beautiful, soft fur, glossy bright eyes, perky ears... And the lump doesn't seem to be bothering him at all, except for he grooms around there frequently. I mean... I remember when he had something similar, and dark bump on his fur. That was months ago... I got him in his ball but forgot all about the lump when diasaster struck and I dropped the ball from standing height. Later, I noticed that lump was gone... But this lump is bigger, and it's a greeny brown color. I dont have barely enough money to take him to a vet, and my mom doesn't think anything is wrong, therefore won't lend me money. :( Please help me! School is starting soon so is my birthday and I don't want to have to be so depressed in such a fun time. :(