Stolen iPhone for 100$ ?

So, actually this happened to me, some co-worker came up to me wanting me open the passcode of his iPhone, he claimed that he found it in his car.
It looked barely used and had no SIM.

I told him I can do it but i ll take time while I actually wanted to try to know who's it's owner.

I called Apple but that didn't help, so I offered him 300$ for it, he accepted.

I paid him 100$ and told him if I open it I ll give you the other 200$, the dude was fired from work and he never came back to ask for his money, I did everything possible to return the iPhone to it's owner but I wasn't lucky with that.

Now what should I do?

"Doing everything possible" would be handing it to an Apple store, for them to return to the real owner.
(Or the Police).

iphones/ipads are linked to the real owners account on Apple's servers (the so-called icloud lock) and are totally useless without that account info.

CONGRATULATIONS!

You are the proud owner of $100 worth of electronics recycle that is not worth $100.

Best path forward: Turn it in at the nearest cell phone store, law enforcement or Apple store. If those don't work for you, chuck it in the electronics recycle pile.