You are confusing an early termination agreement with the consequence of breaking the lease. If you give notice and agree to abide by the terms of an early termination agreement, it does not matter if a tenant moves into the unit the very day you move out -- the early termination fee is the amount you have agreed to pay to get out of the lease, period, end of story. If you break your lease without taking advantage of an early termination clause you can potentially be held responsible for your rent through the end of the lease term.
Further, when you break your lease in a multi-unit complex, you need to keep in mind that your landlord can fill every other unit in the building before placing a tenant in yours and still satisfy the requirement that he mitigate his damages.

