If there was a unilateral as opposed to a mutual mistake concerning the pro-ration of your amount of rent upon move out, ordinarily the tenant is not responsible for the mistake by the landlord if the tenant did not know there was a mistake at the time it happened with respect to the rent issue.If the landlord refuses to give you back your security deposit due to a rental issue, ordinarily security deposits and rent issues are separate and distinct from each other.I would write the landlord's agent a demand letter requesting the return of your money within a set time. If not provided, your recourse would be small claims court.