How to handle the holding fees for move-in?

[Tung Do]

What would be the learning experience in this situation?

Situations:

-I am the owner of my house in Florida, and plan to find a lodger to fill the empty room.
-A person on Facebook messaged me and came over to look at the house. I agreed to give the room to him via message later.
-I explained in writing via message to him that I needed $50 holding fees, which would be non-refundable if he cancelled his plan and wouldn't move in. If he did move in, that $50 would be subtracted from the security deposit.
-He agreed via message, and sent me $50 via Zelle.
-One week later, 1 day before the moving-in date, which was the start of the month, he messaged me saying because he was exposed to COVID, he wouldn't move in, and demanded the refund of his money, which was $50. His reasoning was that he hadn't moved in so landlord was not allowed to keep the deposit.
-He threatened to go to court with me, and would serve me "papers."
-He also threatened to do a "Facebook blast", which he actually did and posted on Facebook that I was a scammer.
-I have screenshots of everything including my conversation with him and that Facebook blast.

Now, I am doing house hacking and will keep looking for tenants/lodgers in the future, and this is bound to happen again and again, so I would love to hear your opinions on this - whether I should do the same or change my holding fees strategy in the future (only stop looking for lodgers once contract is clearly signed, and deposit received). 

What is the best strategy? You find a tenant/lodger, and you want to make sure he doesn't cancel his moving-in and put you in last minute potential monthly rent loss.

In any case, I do plan to appear in court if he chooses to serve me paper, so I can have more clarity about this situation. It will serve me as a reference in the future for other rentals.

If the court determines that I am wrong, I will just pay him back the money.