You're screwed. Landlords have the power once you hand over that key. It's an odd one though. Unless you did intentionally damage the heater (some kind of external damage - I'm slightly suspicious that you haven't used the heater since returning) then I can't see how it is anything but wear and tear. Example: our landlord just paid 15k to get the air conditioning fixed.
What you should have done is get the heater fixed and have landlord pay for it (as someone above notes, it is the LL's responsibility. By ignoring it, you opened the door and the landlord jumped right in.
Lesson learned I hope. Never trust a landlord. Hold back the rent for the equivalent of the deposit and commit to paying for anything you have genuinely damaged. Landlords don't like this? Unlucky. There's nothing they can do about it.
Oh, and don't take cheques. It's 2022.