21. Subject to such conditions, limitations or restrictions, as it thinks fit to impose, the Board ¹[or any other authority authorised by the Board in writing in this behalf] may, in such general cases as may be prescribed by rules or in particular cases by special order, authorise-(a) the delivery without payment of the customs-duties chargeable thereon of goods which are imported only temporarily with a view to subsequent exportation;(b) the delivery without payment of the whole or any part of the customs-duties chargeable thereon of imported goods of such classes or descriptions, as it may prescribe, intended to be used in the production, manufacture, processing, repair or refitting in Bangladesh of goods of such classes or descriptions as it may prescribe; and(c) the repayment in whole or in part of the customs-duties paid on the importation of any goods of such classes or descriptions as it may prescribe, which have been used in the production, manufacture, processing, repair or refitting in Bangladesh of goods of such classes or descriptions as it may prescribe, provided such repayment shall not be made in respect of the class or description of goods for which drawback can be claimed under section 37.