The tort of conversion does not require that you took it, only that you deprived the rightful owner of it.  So, it gets you off the hook for theft, in the sense you aren't liable in the same way as say had you gone into his house and stolen them from him.  But you are liable to him for the tort he suffers by you depriving him of it, which may very well include his legal costs in pursuing its return.

Remember the iPhone lost in a bar?  If your assumption is correct, Gizmodo should have been able to tell Apple, "Sorry, finder's keepers".  But of course, that's not the way it works in California, and that's not the way it works in Oregon either.

Hopefully for you, you're still anonymous.  If your identity is known, this is gonna bite ya in the ass one way or another!  Hopefully you got an extra $10k on top of the 511 BTC you stole to pay for his AND your attorney if and when it finally gets settled.  Best return it now.