What is the ruling of Islamic law regarding the dispute between the seller and the buyer concerning cars that were purchased, a portion of their price paid, and some of them used, after which the seller later sold them without the buyer's knowledge?

We understand that you bought six cars, paid two-thirds of the price, repaired four of them, and the company used two cars for a year due to a delay in transferring ownership. If this is the case, then the sale is void because the cars were mortgaged and the mortgagee did not give permission. You only recover the amount paid, and the value of the investment is prohibited usury (riba). What you paid to repair the cars is not recoverable if you benefited sufficiently from their exploitation; otherwise, you have the right to reclaim the remainder. What you benefited from exploiting the cars during that period is yours, and the company has no right to claim it.