Early termination of lease - penalty of security deposit?

Depends on when you left.
Technically, you should owe the amount of the rent up to the close of the initial contract, unless he's able to re-rent it, because I don't think it's legal to collect rent from 2 different parties simultaneously. If you left prior to Aug 30, I'd say you got lucky.

If he did find someone else to rerent it to without any lost time (ie - you left on 15th, paid for that whole month, landlord was able to get a new tenant to start very next month) I don't think he can keep the security deposit. If there was a lost month (or more) in between, he was justified. Or if the new lease is at a lower rent, I think (am not positive) the landlord can keep the difference between your rents, only for the months of your remaining lease.