(a) The security instrument shall contain a covenant against the creation by the borrower of additional liens against the property superior or inferior to the lien of such instrument, except with the prior approval of the Commissioner.

(b) The covenant required under paragraph (a) of this section shall not apply where a lien inferior to the lien of the insured mortgage is given in favor of a Federal, State or local governmental agency or instrumentality under such circumstances as may be approved by the Commissioner, provided the source of funds for repayment of the inferior lien is limited to surplus cash or residual receipts.