Hi Aamir, and welcome to the forum.
The quoted text below is Fiverr’s recommendation regarding a partial refund (found under the link below, there’s lots of useful info there, provided directly by Fiverr, it’s worth reading the whole thing IMO, as you’ll know how you can handle certain situations “technically” before they even happen, and time can be important, depending on the deadline).
You’ll have one cancellation like that though, but that can’t be helped unless you can convince the buyer to stick with the extra, or maybe offer something else instead that they might have a use for?
You can try opening a ticket with support and hope they can cancel the order without it affecting your stats but there’s no guarantee, it depends on their guidelines and your exact case, I think. But perhaps getting the new order will balance that cancellation somewhat.
Can I partially cancel/refund an order?
Orders can not be partially canceled or refunded at this time. Once an order is created and there is a need to partially cancel/refund your Buyer, we recommend that you ask your Buyer to place a new order. Once that new order is created and then marked as complete, you can cancel the original order.