Showing the house with seller's permission to tenant candidate is perfectly ok.
If the guy will file at court, he has to pay court fee. The fee depends on what kind of court he files. In my opinion no court will take his file. Even if he does, you can easily defend yourself. It will be his loss. I don't think he will really file at court. He has no ground to win or to even interfere your closing.
He may file a complaint to the board against your agent. That may cause some trouble to your agent. However, your agent should be able to defend himself. It will just cost some time to defend himself. If the guy does not exaggerate, the board may even ignore his complaint and do nothing to your agent.
