Honestly I think that is a ridiculous amount of money to require for staying one month extra, especially when you have already been paid for the extra month. You say you want the extra money for delaying your renting process. Do you already have someone lined up for when the lease ends? How long do you think it will take to find someone to rent? I would think having an extra month to already start advertising would actually help you out to find someone and minimize the amount of time you go without rent. You can go ahead and try to get whatever money you want out of tenants. From my own experience, it is doing stuff like this at the end being hard on tenants and nickel and diming them that gives landlords bad reputations. Don't think that doing stuff like this doesn't get around.