It is unlawful to sell to any purchaser any subdivision lot or parcel that is subject to a blanket encumbrance, unless there is a provision in the blanket encumbrance, or in a valid supplementary agreement executed by the holder of the blanket encumbrance, enabling the purchaser to acquire title to the lot or parcel free of the blanket encumbrance upon completion of all payments and performances of all the terms and provisions required to be made or performed by the purchaser under the real estate sales contract. Certified or verified copies of documents acceptable to the commissioner containing such provisions shall be filed with the commissioner prior to the sale of any subdivision lot or parcel subject to a blanket encumbrance.