If a landlord has twenty units, five vacancies, and a tenant breaks his lease, the landlord can fill the five empty units before his obligation to mitigate his damages is triggered in relation to the unit abandoned by the tenant. We have no information about how many vacancies your landlord is trying to fill, even if we assume your proposed replacement will pass a credit check, so we can't tell you if his rejecting the replacement (assuming the rejection was unreasonable) would constitute a failure to mitigate. If you choose a course that results in your being sued, you can try raising and arguing that defense in court and see what happens.