Hi you shouldn't use the contract end date as the exit date. Best to use a different field for that or create you own user specific field (see note 690900). The contract end date is used for financial purposes and explains the behaviour of the reversal posting. Therefor leave the contract end date in place, since that is also the legal end date of the contract and corresponds to the rent that your are charging.
In order to be able to rent out the appartment to someone else you can set the informational indicator in the object assignment of the Object tab.
Regards, Andre