Landlord Charging Me A $500 Fee For Reviewing Applicants

Hi all! Location: New York.

I was renting an apartment in NYC. I found someone to take over my lease, and this person took over my lease and moved in. Contract and lease has been fully transferred over. I have been trying to find someone to take over my apartment for a few months and had shown the apartment to multiple candidates After vetting them and thinking they were qualified, landlord rejected them because they were not qualified.

I later received an email from landlord saying that I am sending people what are not qualified and it has become laborious task to review the applications, and going forward he will charge me $100 to review each application, payments made via Zelle. (This was a few months back)

From my security deposit he is charging me $500 sublease fee, which was in the lease contract and I agreed, but he’s also charging me a $500 for reviewing the applications as “admin fee”. I told him I will only pay the $500 as per the lease, but he’s demanding the additional $500 “admin fee”. He responded “we are not in the business of screening people for sublet. 8 screening is egregious as well. So agree to a reasonable service charge or we will enforce portions of the lease.” (Sounds like a threat to me)

What’s a good response to this and what can the landlord do as I already moved out and a new person took over the lease and moved in? What does it mean by “we will enforce portions of the lease”? Is there a clear case for small claims court?