✔ 最佳答案
Unfortunately, the landlord is legally correct for his/her actions. You will lose in the Land Tribunal if this goes to trial.
Unless you have made prior arrangements with the landlord, security deposit always remains as security deposit. In other word, the rationale of "using the deposit in lien of rent" does not work in the court, unless the landlord explicitly agrees (in writing).
If you have a problem with the security deposit returned by the landlord, you can initiate a legal proceeding against the landlord. But this is not the way to resolve the security deposit.
What you should really do is talk to the landlord back and see what does he/she want in order for the case to go away. Otherwise, you will lose more than just the deposit (as the landlord can go all the way after you).
By the way, your landlord is not a "無良業主". As a fact, you are the "無良租客".