Refund of excess sales tax cannot be withheld merely because revisional proceedings are pending under the statute.

Refund of excess sales tax on reassessment becomes enforceable once the statutory conditions are met, because section 12 creates a mandatory duty to refund amounts paid in excess of the sum due. The "amount due" is the liability as determined on reassessment, and the refund mechanism under the rules gives the dealer a vested claim when the refund application is made and the prescribed notices are issued. Pending suo motu revision proceedings under the Act do not make finality of all possible revision a condition precedent to refund, so the authority cannot withhold repayment merely because revision has been initiated or remains pending.

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Issues: Whether refund of excess sales tax determined on reassessment could be withheld merely because suo motu revision proceedings under the Act had been initiated and were pending.

Analysis: Section 12 imposes a mandatory duty on the Commissioner to refund any amount paid in excess of the amount due. The expression "amount due" refers to the liability already determined on reassessment and does not mean an amount that may later be found payable after further proceedings. The proviso to section 12 and the refund rules contemplate an enforceable refund claim on the making of the application, and the statutory time limits for refund processing do not make finality of all possible revisional proceedings a condition precedent. Once the reassessment showed excess payment and notices in form VII were issued, the dealer acquired a right to seek refund. The authorities were bound to act on the refund application under rules 60 to 63 and could not rely on their own failure to dispose of it before starting revision under section 20(3).

Conclusion: The pending suo motu revision did not justify withholding the refund, and the assessee was entitled to the excess amount.

Ratio Decidendi: Where reassessment determines tax to have been paid in excess, refund under the sales tax statute becomes enforceable on fulfillment of the prescribed conditions and cannot be defeated merely by the pendency of revisional proceedings.

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