Section 21. Refund of excess.

The assessing authority shall refund to the assessee the amount of tax, penalty, interest or other amount, if any, paid by such assessee in excess of the amount due from him. The refund may be made either by cash or, at the option of the assessee by deduction of such excess from the amount of tax, penalty interest or other amount due in respect of any other period: Provided that, the assessing authority shall first apply such excess towards the recovery of any amount due in respect of which a notice under section 8 has been served, and shall then refund the balance, if any.