A producer who has sold corn and has an assessment deducted from the sale price may, by application in writing to the board, secure a refund in the amount deducted. The refund shall be payable only when the application has been made to the board within 60 days after the deduction. Interest shall be allowed and paid at the rate of 6% per annum upon the total amount of such assessment imposed by this Act, except that if any such assessment is refunded within 90 days after an application for refund has been made within the required 60 days after deduction or within 90 days after the first purchaser of corn remits the assessments withheld and deducted to the corn marketing board, whichever is later, no interest shall be allowed on such assessment. Each application for refund by a producer shall have attached thereto proof of assessment deducted.