Ask the landlord in writing for the reason, an itemised breakdown and any supporting invoices or photographs. Check your tenancy agreement and gather evidence such as the inventory, check-in and check-out photos, rent records and messages; deductions should generally relate to unpaid rent, bills or damage beyond normal wear and tear. If the matter is not resolved, you can apply to the Residential Tenancies Board (RTB) for dispute resolution, where the outcome will depend on the facts and evidence. Consider getting advice from Threshold or a solicitor if the amount is significant or the circumstances are disputed.