"security deposit/last month rent" at closing is wrong- now what?

[Pat Robertson]

I recently bought a 4 apartment multi unit for an investment property.  It was fully occupied and the total amount received from seller at closing for "security deposit/last month rent" ended up being short by $800 (one tenants last months rent).  

I learned this after I spoke to each tenant after I took ownership of the building.  Initially, the sellers agent said he would get seller to mail it ASAP, however that never happened.  The seller is claiming that the person he bought the building from didn't give him that tenants last month rent.  

I know its not a lot of money, but it is not my fault that the seller didn't check into this when he bought the building.  Technically, the seller lied on the settlement sheet, he couldn't provide any documentation to prove this tenant in question didn't have her last month rent.  I could not look into this before closing since I did not have access to the tenants, I had to believe what the seller claimed.  

What can I do now?      

Thanks, 

Pat