It's great to hear that you and your client came to a mutual agreement with the canceling of the dispute process. You can ask your client to accept your dispute request. Once he accepts, you can refund a partial amount from the job/contact page.
I checked this and see that you are communicating with our team regarding this case on your existing ticket. Please allow time for our team to respond and update the ticket. If you have any additional question please post them on that ticket for further assistance.
Thank you for your patience!
If it's already gone to mediation, let the mediation person handle it because if you refund there is no guarantee you will get a payout from the dispute. Let the mediator dish out the money.