(2) On a sale of land held by trustees, the trustees may, where the proceeds are liable to be invested, contract that the payment of any part, not exceeding ⅔, of the purchase money shall be secured by a vendor's privilege or mortgage of the land sold with or without the security of any other property, such mortgage or vendor's privilege, if any buildings are comprised in the mortgage, to contain a covenant by the mortgagor or purchaser to keep them insured against loss or damage by fire to the full value thereof.