Self-assessment obligation requires payment within a set period and may attract a capped penalty if unpaid. Section 140A requires an assessee who furnishes a return under section 139 to pay the tax shown as payable, after credit for tax already paid, within thirty days if that tax exceeds a specified threshold. Payments made under this self-assessment are deemed to have been paid against any subsequent provisional assessment under section 141 or regular assessment under sections 143 or 144. Failure to pay within the period renders the assessee liable to a penalty determined by the Income-tax Officer, subject to a statutory cap and the requirement of a reasonable opportunity of being heard, unless an assessment is made before the expiry of the thirty-day period.