Something similar has happened to me twice (different scenarios), and the only commiseration I can offer is that they are life's learning experiences. Now you know a little more to get everything mentioned down on paper before you sign. I find it hard to believe that the referral bonus wasn't mentioned in an email or other notification-level policy statement, but aside from that you're probably just going to have to…
FTA: "I was entitled to a $10k referral bonus offered by the CEO through internal emails." It was mentioned in an email.
One of my life lessons was that I didn't save email from one of the scenarios, email in which the CEO had asked me how old I was.