How legal is a verbal agreement with a landlord and a resident?

I recently moved into a new townhome and before I moved in, my landlord (who also happens to be a realtor) requested along with all of my deposits, a post dated rent check for the first months rent. He assumed me it would not be cashed until the first, as I would never have given him a check for money I didn't have had I thought he would cash it. He ended up cashing all the checks at once! He said it was an honest mistake, which I believe. Now it seems he does not want to pay for the overdraft fees I had even though he acknowledges his responsibility. I have already spoken with my bank and there really is no such thing as post dating a check. Is he liable since we had a verbal agreement?