Domain Mapping is part of our Domain Registration package and cannot be refunded separately.
You are welcome to keep your domain with us and update the nameservers to point to another hosting provider by following this guide:
http://en.support.wordpress.com/domains/change-name-servers/
After 60 days from the date of purchase or registrant contact information change, you may also transfer your domain to another registrar by following this guide:
http://en.support.wordpress.com/domains/transfer-domain-registration/