Why would you assume for there to be no proper conclusion? It is not like running away from the truth helps anyone. Is it not obvious that anyone would inform of payment delays, non-payment against dissatisfactory services, etc? What’s there to gain by not informing? It is not like these things don’t happen, because they do happen. Often more times than you did like.
I would like to take this claim at face value and just assume it to be true that a payment was not made. If its a troll, it would be a really sad one. Going forward under this assumption, the freelancer certainly thinks that he/she should have got paid for the services delivered. This is where a communication gap could exist. I highly doubt we have not properly closed an engagement. It is more likely it was closed against the interest of the freelancer, and the freelancer won’t take NO for an answer, and is now making a claim under an incorrect pretex.
What would you advise as the best course of action in such a scenario?