Withholding tax compliance determines deductibility of business expenses; unpaid source tax blocks deductions until properly paid or deemed paid. Tax payments and related surcharges are not deductible for computing business income, including domestic tax, employer-paid tax, and foreign tax eligible for relief. Deductions are disallowed where withholding tax under Chapter XIX-B has not been deducted or paid by the due date, subject to allowance when such tax is later paid or deemed paid on the payee's return. Payments from employee funds are nondeductible unless withholding arrangements exist, and specific limits and authorisation requirements restrict deductions for partner remuneration and interest, with defined concepts of book profit and working partner.