Mo. Rev. Stat. § 100.820

An agreement between the board and an eligible industry shall provide that all or part of program costs are to be met by receipt of assessments. Assessments shall be based upon wages paid to eligible employees. If business or employment conditions cause the amount of the assessment to be less than the amount projected in the agreement for any time period, then the employer shall pay to the board the amount of such difference, then a portion of withholding tax paid by the employer pursuant to sections 143.191 to 143.265 may be credited to the board by the amount of such difference. The employer shall remit the amount of the assessment to the board. When all program costs, including the principal of, premium, if any, and interest on the certificates have been paid, the employer credits shall cease.