Deduction of tax at source: unpaid withholding requires assessing the taxable proportion of nonresident remittances to fix payer liability. When tax has not been deducted on payments to non-residents, the tax officer shall determine the appropriate proportion of the remitted sum that is chargeable to tax to quantify the payer's liability, based on the nature of the remittance, income components and other relevant facts; failure to deduct renders the payer a deemed assessee in default with interest consequences.