It is always your cash back... if they TRY to offer you store credit (they have to, its their job), just say nah and they'll have to return ur money! It's the store policy, they can't refuse you! They will try to offer you other games tho... altho in melb cbd they know me for "hiring" games, so they just straight away gimme my money back...
PS. You don't need to give bullsh!t excuses either (ie. It didn't work). Just say you didn't like it if they ask whats wrong with it... works like a charm...
PPS. I think they will soon put me on the "banned from 7 day return" list (if one exists), I once repeatedly returned 5 times, but the 6th time I kept the game... so I guess that's all they are after...