I got a job in another city and now I'm paying rent on two apartments?

This is a common bind, and the answer depends on your lease terms and your landlord. Subletting in Korea legally requires the landlord's consent; without it you can be in breach, so do not sublet quietly. Talk to your landlord first, since some will agree to a sublet, especially short-term, in writing. On breaking the lease early, a fixed-term lease does not automatically let you leave, and the common practice is that the departing tenant helps find a replacement tenant and often covers the new agent (brokerage) fee, after which the landlord returns your deposit. Many landlords are reasonable about this because they get a new tenant without a vacancy. If you are on jeonse or have a deposit, you generally get it back when the replacement moves in, not necessarily on your move-out day, so plan your cash flow. Check your contract for any special early-termination clause. The cleanest route is usually to tell the landlord and agent now, agree to find a replacement and cover the agent fee, and get the deposit-return timing in writing. If the landlord refuses to cooperate, a free legal aid center or the Korea Consumer Agency can advise.