New York Consolidated Laws, Real Property Law - RPP § 227-e. Landlord duty to mitigate damages

In any lease or rental agreement, excluding any real estate purchase contract defined in paragraphs (a), (c) and (d) of subdivision four of section four hundred sixty-one of this chapter, covering premises occupied for dwelling purposes, if a tenant vacates a premises in violation of the terms of the lease, the landlord shall, in good faith and according to the landlord's resources and abilities, take reasonable and customary actions to rent the premises at fair market value or at the rate agreed to during the term of the tenancy, whichever is lower.  If the landlord rents the premises at fair market value or at the rate agreed to during the term of the tenancy, the new tenant's lease shall, once in effect, terminate the previous tenant's lease and mitigate damages otherwise recoverable against the previous tenant because of such tenant's vacating the premises.  The burden of proof shall be on the party seeking to recover damages.  Any provision in a lease that exempts a landlord's duty to mitigate damages under this section shall be void as contrary to public policy.