Multi-fam property. Difficult tenant. How to get appraiser access

[Cee Williams]

I found a 2-4 multifamily property in Brooklyn that I'm interested in purchasing via a mortgage. The problem is that one of the units is currently occupied by a difficult tenant. The tenant doesn't want to give access to the their unit. This is going to make it impossible for the appraiser to come do the appraisal. An all cash offer is not a possibility for me and I don't have the 10%-20% down payment amount to do a hard money loan. I'd like any insight on how to go about this purchase. Is the tenant required by law to give the access to their unit if the owner is selling and needs access to the unit for the appraiser to asses and conduct the appraisal for the buyer? Is this something that is only allowed if it was written in the lease? Does it make a difference if the tenant is on a lease, month to month, paying or nonpaying?

Any experienced real estate lawyers or real estate professional out there familiar with how to navigate this situation?

[Nathan Gesner]

Quote from @Cee Williams:

It's simple: the law requires you to give notice of intent to enter; it does not require you to obtain permission.

The Seller needs to give the tenant notice that the appraiser is coming in on [DATE] at [TIME]. The Tenant will either let them in or the Seller will open the door. If the key has been changed, then a locksmith will open it and the charges passed on to the tenant. 

It may help to include a copy of the law in the letter. I would also let tenant know that failure to participate may result in termination of their lease or eviction.