It shall be lawful for the Director General, if it is proved to his satisfaction that any sales tax or penalty has been overpaid or erroneously paid under this Act, to order the refund of the sales tax or penalty that was overpaid or erroneously paid subject to such limit as may be prescribed by regulations made under this Act:

Provided that -

(a) no such refund shall be allowed unless a claim in respect of it is made in the prescribed form within one year after the overpayment or erroneous payment was made; and

(b) the Director General may reduce or altogether disallow any refund due under this section to the extent that the refund would unjustly enrich the claimant.