I once was also in a similiar situation. Since you're only subletting a room and not the whole appartement, then you can terminate the contract within two weeks. Since you don't have a binding written agreement, there is no way for them to claim that you must pay any additional time or period, if you're out - you're out and you don't have to pay any additional money. They have to pay back the deposit, but you must allow for a period of up to three months. If they don't pay back within three months, then you should engage a lawyer, respectively join the Miterverein and engage them.
Apartment Rentals