It would probably be easier if you have a few words written down in Korean, such as "receipt" etc.

Just one thing regarding the receipts. They might ask you if you need a "cash receipt" as opposed to just a regular "receipt". A cash receipt (sounds like 'hyunggum yungsujeung') will involve you giving them your Korean registration number etc., which is really only something for people paying taxes in Korea. But a regular receipt (sounds like 'yungsujeung') is what you will want.

T-Money card, if you say it slowly, I am sure they will understand what you want. I haven't tried to refund any portion of my T-Money card, but I usually do not keep much money on the card anyways. As it is very easy to top up the card at almost any subway station, perhaps it may be easier to keep a bit of credit on the card just in case you return?