Pay us even if it's our fault

Our electricity service providers recently overcharged us and admitted to it because of a new tenant that was using electricity via our meter until they installed a separate meter. They said that we would receive a credit once they know the average consumption of the new tenant but upon inquiry this week they changed their tune. They said that we would have to get the money out of the new tenant ourselves despite their overcharge of electricity consumption. Furthermore there had been a computer error on their side for the account in which they decided to recover money's not collected for months without a prior communication for incorrect billing on their side. They seem to be a law unto themselves and have no care for customer service & liason. They just demand their money's even if they are at fault.