Actually, normally it is not permitted to cancel the return half of an award trip and get half the miles back.
http://www.united.com/web/en-US/cont...vel/types.aspx
You can also book a One-Way Award if you wish to travel one way. If you redeem a roundtrip award and only travel one-way, the unused portion of the award will be forfeited.
And since you have a stopover involved they almost certainly won't give you the refund.