Check with a lawyer, this varies all over the world and you don't want to mess this up. Not only might the money become theirs again, it might become theirs with interest. Though you might be able to charge them storage fees. don't forget that laws change so keep in touch with your lawyers so they notify you when laws changes so you can remain complaint. Make sure you keep record of what the lawyers say as well - if (when?) you mess something up record of trying to do the right thing can work to your favor in court.