It is unlawful for a manufacturer to sell or contract to sell motor vehicles to a retailer on the condition or with an agreement or understanding, either express or implied, that the retailer shall finance the purchase or sale of motor vehicles only with or through a designated person or class of persons or shall sell and assign the conditional sales contracts, chattel mortgages or leases arising from the sale of motor vehicles only to a designated person or class of persons, when the effect of the condition, agreement or understanding so entered into may be to lessen competition, or tend to create a monopoly in the person or class of persons designated.