Sure, you can delete a domain, but it is probably that you can not get any refund for it because they are all over 5 days old. You can see more information about domain deletion from here: https://www.dynadot.com/community/help/question/refund-domain
Please notice that there are chances that the domain get registered by other people during the process. So, there is a better way for your reference. You can initiate a domain ownership change within Dynadot accounts after you two agree on the transaction. Here are the steps for domain push: https://www.dynadot.com/community/help/question/domain-push
Please be aware that we will not involved in your private transaction. You will need to contact with your client for further negociation.
Please feel free to contact us again if we can be of any further assistance.