It depends on the leasing company how they handle the security deposits. Some leasing companies ask for a new deposit from the lease buyer (in this case, the previously obtained security deposit will be returned) while others allow for the transfer of the deposit along with the lease. Also, not all car leases require a security deposit. However, how the deposit will be handled is also a point of negotiation between the seller and buyer. Make sure that you are fully aware of the policies of the leasing company whose car lease you are assuming.