Do I have a case in getting my security deposit back at the small claims court?

A security deposit is to protect a landlord from damage. It is not to be used a penalty for breaking rules. If you did not cause any damage to the apartment you should get the deposit back.

As long as your installation and removal of the temporary wall caused no damage to the apartment, you are entitled to the return of your security deposit. The purpose of a security deposit is to protect a landlord from damages that might be caused by the tenant. if there are no damages when the tenant moves out at the end of the lease, then the security deposit should be promptly returned. In order to pursue this matter you should send the landlord a letter -- the original by certified mail and a copy by first class mail with a certificate of mailing. You should demand the return of your security deposit within 10 days. If the landlord does not return the security deposit after 10 days have passed you should sue to recover it in small claims court.

I'm not so sure I agree with attorneys Bianchi and Rothstein. If I represented the landlord, I would argue that impermissible changes are akin to damages inasmuch as the security can be used to restore the unit to it's original undamaged/unchanged condition. You receiving a text from the broker, who probably did not have the authority to bind the landlord, is probably not sufficient authorization to make structural changes that I assume your lease did not allow without landlord's consent. If there is any chance that the restoration would cost more than the deposit withheld, you might be wise to drop it. Just my two cents. Good luck!!