Help!

Hello dear members, I urgently need advice from experts. Hope you can help me here.. A few days ago I had a driving accident (I am the “accident causer” in this case). It was really only a light “stupper”. Neither with my vehicle nor with that of the opposite person can be recognized from outside damages (photo I have). I didn’t come to call the police because I thought there would be nothing (my first accident since my driver’s license 2007 The other driver (older gentleman between 40-50) did not insist on it either. Only data were exchanged. Now the Lord claims that with him the “heating tank is dented and some foam torn” (can not imagine anything under it). With insurance it would cost us allegedly 1200€, if we settled this privately, he insists on 300€. I feel quite fooled. How can it be that inside the car is a bit dented if you don’t even know from outside. How should I proceed? Does it make sense to use the police afterwards? Hope for advice soon. Thank you!