Fiverr has metrics that need to be achieved in order to push sellers to be excellent sellers. If a buyer asks you to cancel a completed order, just say “no”. You did the work, you deserve to get paid for it. The more you agree to cancel, the more your cancellation rightfully goes down. If you don’t want the cancellation rating to go down, don’t agree to cancel completed orders. In this situation, he will leave a negative review which also spoil the seller profile. How can seller avoid this. And also refuse to cancel the order.