21. The sale shall be made in accordance with the Rules of Sale for the time being in force under the Civil Procedure Law, Cap. 7, and, unless the Court otherwise directs, the net proceeds of the sale shall be applied in the first place to the payment of the trustee’s costs occasioned by the application, and in the next place to payment of the amount due to the mortgagee for principal, interest, and costs, and the surplus (if any) shall then be paid to the trustee. But where the proceeds are insufficient to satisfy the amount due to the mortgagee, he shall be entitled to prove as a creditor for the deficiency and receive dividends thereon rateably with the other creditors, but so as not to disturb any dividend then already declared.