Is the failure to return a security deposit after verifying violations considered theft, and what is the punishment for such an act in the Qur'an and Sunnah?

If the insurance amount paid is due from the tenant because of his negligence or transgression in using the car, then the employee's failure to return the amount is not considered theft in the Sharia sense. Rather, it is procrastination in returning a right and seizing the property of others. The perpetrator is sinful, and the aggrieved party has the right to complain against him and sue him, and he deserves to be disciplined by the ruler.