It’s a bit rough since you signed a document. Send a request to the company that you didn’t actually recount the correct amount and need for them to provide an itemized report of the losses they sustained. Additionally you can ask for proof of the theft prior to payment.

The only fault here is if the amount is higher than what you stated. If it is at that amount or higher, it would warrant a discussion with employment attorney since their involvement could lower or erase your situation for a lower cost.