I'll preface by saying I've been employed as a claims adjuster in Illinois for 14 years handling auto, liability and property claims and here is my free opinion on your situation without knowing all of the facts. You are dealing with 2 separate issues here; Legal liability and contractual liability.
1) Legal liability: The Thrifty vehicle was rear ended by another driver while being operated by your employee. The other driver would be guilty of the tort of negligence; and in the absence of any contributory negligence on the part of your driver, the other driver would be responsible to pay for the property damage to Thrifty's vehicle and any medical expenses, wage loss or pain and suffering to your employee from accident related injuries. The other driver did not have insurance and dealt directly with Thrifty on the property damages. We do not know why the other driver only wrote a check for $4K but he included release language on the check. By Thrifty cashing that check I believe it is sufficient to stand as a release against the other driver for any further property damages. What that means is he is now released from any further liability for property damages from that accident and I do not think you could legally pursue him for repayment of the additional $2K Thrifty is looking for; if you do pay Thrifty any additional funds. You may be successful in convincing a judge that you are now the injured party and not Thrifty; but I doubt it.
2) Contractual Liability: Assuming you did not purchase any insurance/damage waivers from Thrifty when you rented the vehicle, you signed a contract when you rented the vehicle indicating that your personal insurance coverage would be primary to cover any damage that would occur to the rental vehicle while it was in your possession. Thrifty is now saying that you are contractually obligated to pay for the damages which total $6K. They received $4K and now they want $2K from you.
I honestly don't know why your insurance company would tell you to pay this but again....I don't know all of the facts. If this were covered by your insurance policy you would only be responsible for the collision deductible amount. I would tell Thrifty that they concluded the property damage claim directly with the tortfeasor(negligent party) and their cashing of the check which included the release language not only concluded the property damage claim IN ITS ENTIRETY but also predjudiced your rights to pursue repayment from the tortfeasor because he has now been released. See what they say. Worst case scenario....try and negotiate the amount they allege that you owe and get a copy of the damage estimate if you have not yet been provided one.
I hope this helps and good luck with your situation.
