A rent concession is made, within the meaning of this Act, when, in case of a written lease of real estate or a part thereof, the lessor before or at the time the lease or any agreement therefor is entered into, and in consideration of such lease or agreement therefor, directly or indirectly, gives, or agrees or promises to give, to the lessee, without express mention thereof in the lease, any of the following: (1) any credit upon the rent reserved by the lease between the parties, or rebate of such rent or any part thereof after payment thereof by the lessee, or (2) the right, privilege or license to occupy the leased premises for a period other than the term created by the lease, rent free or for a rent less than the average rent fixed by the lease for the entire term, or (3) any other valuable thing, right or privilege. Repairing and decorating the leased premises by the lessor shall not be deemed a rent concession. An agreement by a lessor to waive any of the terms or conditions of the lease other than those relating to the payment of rent shall not be deemed a rent concession.