Seven

If that's related to the order and do not contain your personal information, then it would be okay mostly.
Definitely, you can help your buyer to make everything easy by maintaining the TOS.

I suggest you to discuss with your buyer, share them the working system and show them how you made this clear in your gig details.
Offer them to rework on it for additional tasks or paid revision (may be you offer them less budget than usual for better business relationship).
I hope they can understand.
If not then it's up to you. if your gig details clear that you don't provide any revision/no revision once the sketch is approved, you can deny refund request and if needed reach customer support.

But again! I appreciate to discuss with buyer in details and make a better business relationship.

Hi Pradeep,
I can understand your frustration. I also believe that it's important to have discussion before starting a project.
Still, I suggest you to keep the conversation within the Fiverr platform.
As an alternative solution, I can suggest you to offer a custom offer to discuss, as you will get video call option (powered by zoom) in your order page.
Also, I am happy to let you know that, Fiverr is working on to avail the video calling option for all users within the message before placing an order.
It's in trial stage now and will be available to all soon.

Make sure you followed his requirement perfectly.
Did you submit the work sample before the delivery and you got his approval?
You may politely offer him revision if needed.

Finally if you think he is playing with you though you've no fault with your work, simply decline his request and contact fiverr customer support and share all the details about the game the buyer is playing with you.

Fiverr support team will review and if they find you right, they may will support you to overcome the scenario. Even if the order get cancel, they can help you to avoid negative impression on your completion rate and may be compensate with the same value.

So before reaching customer support make sure you're right.
 
Additionally, you can directly reach to support and share the details and ask for cancellation from their end and help you to avoid decreasing your completion rate. They will help you.

Fiverr offers Online Tutor service under Lifestyle category.
Lesson category allow video call feature within the fiverr platform powered by zoom.