Moving out of a rental flat outside of the contractual notice period is possible if one finds a suitable replacement renter. This new renter must be able to afford the flat and be prepared to take it over under the existing terms and conditions.
However, if the replacement tenant backs out shortly before signing the rental contract, the search starts all over again. For according to the law, the existing tenant is relieved from his responsibility as renter only after the new tenant has signed the rental contract. There is then not always time to find a suitable replacement tenant. In this case the existing renter is responsible for paying the remaining rent until the regular notice period comes around.
It is therefore better not to let it get so far and from the beginning to suggest multiple acceptable potential renters. It is also good to have these interested parties sign a statement that they would take over the flat under the existing terms and conditions. Should the landlord not respond to the recommendations or dismiss them without grounds, this then serves as proof that the renter had found an appropriate replacement within a reasonable amount of time. A landlord has 30 days to evaluate potential renters. If the landlord does not respond within this time period, the renter is freed from the contract on the day on which the new renter would have taken over the flat.