Refund procedure: excess tax may be refunded or carried forward subject to conditions, security and filing requirements. Excess tax, penalty and interest paid may be refunded after first being applied against other dues under this Act or the Central Sales Tax Act; the balance may be refunded or carried forward as tax credit at the dealer's election, subject to ongoing audit or information requests, the Commissioner's power to demand security as condition of payment, exclusion of time taken to furnish security or information from prescribed refund periods, safeguards for sales to unregistered purchasers, reassessment risk where buyers claim credit, and denial of refund where required returns are not filed.