You have agreed to it, so you cannot. You are arguing backwards, you should have argued the point prior to accepting any settlement. The question then is "why buy a car back you already own?" If the repair cost were less than the market value, the car was not a total loss and you were entitled to the cost of repair. If the repair cost is more than the market value, then the car is a total loss and you have agreed to settlement less the salvage value.