Deemed tax deduction: failure to withhold treated as deduction on filing of payee's return where no default attaches. A proviso deems that where a payer fails to deduct tax under the Chapter XVII-B withholding regime but is not treated as an assessee in default under the first proviso to the default rule, the payer shall be deemed to have deducted and paid the tax on the date the resident payee furnishes its return of income, for the purpose of determining deductibility under section 40(a)(ia).