A private contractor gave me an estimate to replace my air conditioning unit and I decided not to do it, because he never show up to installe it the day that I told him. The contractor went to the realtor that managed my property and told them that I decided to do it (without any contract signed or my permission). The contractor replaced the unit without my knowledge or any contract signed. Noiw that I am trying to sale my house, I came to find out that the contractor putted a lien on it without saying anything to me or having anything in writting.
I need to know if this is legal and the requirements to put a lien in somebody else's house without them knowing anything about it for something that it wasn't authorized by the owner.

That sounds very frustrating! You would have had to recieve notice before a lien could be obtained. I suggest you speak to an attorney asap regarding your legal options.

See also Alabama statues on liens: http://www.legislature.state.al.us/c...975/136141.htm