He isn't required to accept the person you find.
He is required to be diligent in mitigating any damages caused by an early exit, planned or not.
The time to have asked that question is before you signed the lease.
The legality of any issue is only adjudicated upon a trial.
Proclamations of this being legal, that being illegal, are simply speculative, not definitive.
Changes to a lease are critters of contract law, not statutory fiats, although statutes can be binding on the adjudication of many disputes.
Take murder for instance, prohibited in every square mile of the USA, yet murders still occur.
Countries have laws about entering illegally, one has only to look left or right to see that sovereign borders of some nations mean nothing to some people.
On the other hand, very few people break into or out of the Democratic People's Republic of Korea, do they?