What if I volunteered to find the girl's severed finger because I wanted it for a souvenir?
That sounds so disgusting. Why would you even want this?
I wanted to play a joke on a friend who has a big crush on me by wearing a fake engagement ring on a fake finger.
If you are looking for someone's finger the doctor's may still be able to reattach it. You should turn it in if you find it so the girl has a chance at keeping her finger.
I would rather keep it. It would be of more use to me this way.
It's wrong to keep a severed finger if you are helping to search for it. This is a serious situation and you should not use it to your advantage. Turning it in would be the right thing to do.