I've read this book maybe a year ago in German and it wasn't much older than a few years at that point.
The protagonist is a boy in high-school who is fascinated by dinosaurs. He is always trying to dig up fossils in the mountains that are behind his house where he lives with his sick mother and their cat.
He is working somewhere and is the normal average nice boy you find in stories. Never doing anything bad. But while working he steals a little figurine that his mother would probably like. He doesn't have the money to just buy it.
After that, while digging in the mountains, a big hole opens up. He doesn't think too much about it, but when he returns home and goes to watch some TV in the basement a devil is sitting on his couch. The devil starts to threaten him that he will kill his mother if the boy doesn't bring him - snacks. And he wants a house of his own.
The boy realizes that there is not much he can do against the devil and so he starts to steal food, because the devil can only eat stuff that has been stolen.
Because he is pretty helpless he tries to find someone who could help him. And his only idea is the gothic-girl he has seen in school a few times. Someone said she summoned demons a few years ago. Of course she denies that when he asks her about it.
Fast-forward: she really summoned a devil accidently before and it nearly got her and her family killed.
Together they try to find a way out, but the boy is getting lured by the devil everytime they are close to their goal, for example by promises of healing his mother.
He manages to do the thing that the devil wants to heal his mother - but sadly the process of healing means that a stadion came crashing down while a public game was in progress and so there is someone who died and his heart could be transplanted.
In the end they find out that the devil has one weakness - cats.
And with the help of his house-cat the two of them manage to make the devil flee. The Happy End is that they are now regularly sending devils back to hell with the help of the cat.
Can you help me remember what the name of this book was?