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Without any contract, it would basically become a battle of he-said she-said. Courts tend to favour the tenants quite a bit too.

So might be worth at least checking with a lawyer familiar with landlord-tenant stuff to find out if it's worth pursuing in court, though if the 6 months was verbally agreed to losing just the security deposit might be better than taking the risk that a judge decides you're responsible for the extra 3 months, or splits the difference and says 1 1/2 months or something.