The commissioner may by order deny or revoke any exemption specified in section 51-19-04 with respect to the offer or sale of a specific franchise. No such order may be entered without appropriate prior notice to all interested parties, opportunity for hearing, and written findings of fact and conclusions of law, except that the commissioner may by order summarily deny or revoke any of the specified exemptions pending final determination of any proceeding under this section. Upon the entry of a summary order, the commissioner shall promptly notify all interested parties that it has been entered and the reasons therefor and that within fifteen days of the receipt of a written request the matter will be set down for hearing. If no hearing is requested and none is ordered by the commissioner, the order will remain in effect until it is modified or vacated by the commissioner. If a hearing is requested or ordered, the commissioner, after notice of an opportunity for hearing to all interested persons, may modify or vacate the order or extend it until final determination. No order under this section may operate retroactively. No person may be considered to have violated section 51-19-03 by reason of any offer or sale effected after the entry of an order under this section if the person sustains the burden of proof that the person did not know, and in the exercise of reasonable care would not have known, of the order.