I've stripped tons of Xbox Live Gold from accounts, I'm fairly good at this, I do a call-in method. However Quad is the one who showed me, but he'd probably do it for you and not give his method.
Just have them transfer you to billing. Say your going on vacation and you dont want your XBL to go to waste. Ask if the can cancel it and then send you some back.
Yeah just call MS if it has a CC on it works the best just cancel the subscription they email the remaining balance.
Or
Call them and tell them your leaving for the ARMY and would like them to token your remaining balance done deal if they say they can't tell them to speak to a supervisor explain your in the army and they'll do it for you.