I realize that you are looking for me to show you a law that can contradict the police in their position that the toolbox must be returned to the employee. I cannot do so, because no such law exists. If you want to retain the toolbox, then you will have to sue the employee and ask the court to grant a preliminary injunction permitting you to retain in in anticipation of receiving an equitable lien or constructive trust on the box, to cover the employee's debt, created by the fraud against you and the customer.
Absent the above-described action, you must return the toolbox. As for the storage charges, if the police are okay with that, and you want to charge the employee for storage before returning the toolbox, then go ahead. However, in my opinion (and, I realize you won't like this answer either, but I don't want you to get into trouble), until the employee has notice that he must pay rent on the storage of his property, you cannot claim a lien on the property for that storage.
Bottom line, if you want to stop the employee from taking his stuff, then you need a court order.
For a civil litigation attorney referral, see this link.
Please understand that I "justanswer" questions "about" the law. I have no interest in providing you with anything less than a completely satisfying answer. However, if the law does not favor your unique circumstances, then the best that I can do is to explain what the law "is" and what it "is not."
Hope this helps.
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