S. 43B : Deductions on actual payment-Interest payable to Financial Institutions-Rehabilitation plan and accepting debentures in discharge of outstanding interest-Explanation 3C, cannot be invoked-Interest is allowable as deduction-Interpretation of taxing statutes-Retrospective provision for the removal of doubts Cannot be presumed to be retrospective if it alters or changes law as it stood-Ambiguity in language to be resolved in favour of assessee. [S.43D]