It will take evidence. If the only evidence you have is your word versus theirs, then it becomes a swearing contest and the judge will have to try to decide who is telling the truth. But you indicated that you have documentation of the work you did, you may have correspondence or evidence of other exchanges with the landlord which reflect the agreement, and their present demands may well reflect an admission that they were giving you some credit for the work you performed. So... keep looking for evidence that will help your case.