What is a very sad possibility is that it was all planned from the start. If John did well and the payoff was more than the $300 investment, then he contacts you right away and says here is the $300, give me my cut, we had an agreement. Or he just tells you to take your $300 out of his cut.
If John did not at least win back his $300 investment (which you fronted), he uses whatever excuse he needs to avoid paying you, and had a "free" shot at making some money with no risk to himself.
Regardless of his reason for not paying, even if it was being unable to pay at this moment due to unforeseen circumstances (highly unlikely), he has proven to be a total scumbag in the way this was handled. Even in the most understandable of circumstances, like if your house burned down and you lost your job the same day you were supposed to pay, you still go to the person you owe and be honest about what is going on, and work out a payment arrangement to the absolute best of your ability. Everything he has done in regards to this situation has shown that he is unwilling, not unable, to pay. He stole $300.
