It shall be unlawful, except as provided in Section 11013.2 or 11013.6, for the owner, subdivider, or agent to sell or lease lots or parcels within a subdivision that is subject to a blanket encumbrance unless there exists in the blanket encumbrance or other supplementary agreement a provision, hereinafter referred to as a release clause, which by its terms shall unconditionally provide that the purchaser or lessee of a lot or parcel can obtain legal title or other interest contracted for, free and clear of the blanket encumbrance, upon compliance with the terms and conditions of the purchase or lease.